These items could be artifacts that have shaped the course of a culture, relics that hear the power of the divine within them, they could be strange technologies, sacred bones of martyrs...it really doesn't matter.
What matters is that these items have some kind of special effect, and while there may be a few of them present within a setting, they more powerful items of power become exponentially less likely to appear in a game.
The first thing that you need to consider is what type of effect these items might have. Do they allow a character to advance one of their abilities beyond "regular human limits"? Do they provide access to an ability to most characters would not be able to possess normally? Do they impair the people around them, while keeping the wielder at full strength?
Each of these options tells something very different about the item of power.
The second thing to consider is how far the exponential scale should work for these items. If the characters have attributes rated from 3 to 18 (a la D&D), and an item allows them to add a single point to their existing attribute how much does this mean...it certainly means much less than adding a single point in a game where the attributes are rated from 1 to 5 (a la White Wolf).
With this in mind, how many scales of power apply to items of this type? Are there only three levels (weak, moderate, powerful, each adding an additional +1 to a attribute or skill, or providing 1 extra cumulative ability), or are there twenty levels of artifact carefully catagorised by scholars and mystics...
Personally, I'd recommend the number of levels in the scale to match the skill or attribute range for the characters.
Die step systems (d2, d4, d6, d8, etc) would count each step as a single level of attribute, where the item might allow an additional roll to be made. This could be used instead of a character's normal die roll, combined with the die roll or perhaps some other option depending on the games existing mechanisms.
Lets run for the moment with 3 hypothetical level systems.
- 3 Level (weak/common, moderate/uncommon, powerful/rare) - Probably used is a system based on d6's, where a +1 bonus can be very beneficial. Or could be used in a system where unusual mechanisms and modifiers aren't commonly a part of the system, even the small bonuses provided by such items a highly sought commodities.
- 5 Level (negligible/very common, weak/common, moderate/uncommon, powerful/rare, legendary/very-rare) - Probably used in a system with dice pools, where a +1 could represent extra dice to roll, or could allow bonuses to the roll of a d10. In such a system, the lowest level items would be finely crafted tools and it is only when reaching into the higher level items that mystical phenomena might become clearly involved.
- 10 Level (1st degree, 2nd degree...10th degree) - Probably used in a system incorporating d20 mechanisms or even percentile dice. Each degree adds a point of bonus, which isn't much, and doesn't distinguish a lot between the different levels of power within the items.
Now that we know the levels and the benefits that each level applies, we can start looking at the factors which make and item powerful.
I like using exponential scales, because they mean the more powerful items become harder to find. There are half as many uncommon items as there are common items, and half as many rare items as uncommons. The specific setting could tweak this even further...1 in 10, 1 in 100.
Naturally, the more levels in the scale, the less difference there will be in the scarcity of item availability. For example, if there are 10 levels at ascending degrees that halve the availability, there will be 1 "level 10" item for every 1024 "level 1" items. Once you start dividing the numbers by 3 or 4, the rarity really increases for those high level items. Personally I think that's a good thing...especially after too many monty hall campaigns.
But what imbues the items with power? How is their level determined?
The following is just a suggestion for some tables that could be used to calculate different aspects of item power. A GM could use any one of these, or they could add together a couple of aspects then divide by a regular amount to generate an items power level with respect to the campaign setting. Each of these could be weighted differently depending on the settings specific attitude to items of power. you might want to double the amount of energy gained from an item's age, or maybe divide it's fame factor by 5. They're just ideas.
This could be the physical age of the item, it could be the time an item has spent in a sacred grove absorbing mystical energies, or turn the timeframe into days or weeks and it could be the time spent crafting the item.
1 yr = 1pt, 3 yrs = 2 pts, 10 yrs = 3 pts, 30 yrs = 4 pts, 100 yrs = 5 pts, 300 yrs = 6 pts, 1000 yrs = 7 pts, 3000 yrs = 8 pts, 10000 yrs = 9 pts, etc...
Mystical items could gain power from the desire of the people around them, the more people want an item, the more powerful it becomes; the more they are willing to do to get hold of it, the more powerful still.
1 pt per 10,000 people who would like to get he chance to see or touch the item.
1 pt per 1,000 people who would like to add the item to their collection.
1 pt per 100 people who would hurt someone to get hold of the item.
1 pt per 10 people who would kill for the item.
Any of these people in the immediate vicinity, would have their desire value magnified. This would come from a metagame effect because people nearby are more likely to cause narrative conflict for the owner of the item. It also has the in game effect that the desire of a supernatural connection that will probably link between the item and people's souls...they are more likely to take risks when the item is nearby...and the more risks they are willing to take, the more energy the item can draw from them.
How well is the item known?
1 pt = Known by local scholars and occultists.
2 pts = Known by most locals and throughout the regions by scholars and occultists.
3 pts = Well known in local area, known as an article of trivia by people across the country.
4 pts = Known across the state or province, occasionally mentioned nationally, some global impact.
5 pts = Household term throughout state or province, well known across the nation.
6 pts = Well known globally.
Other factors might include...use in key historical events (1 pt for involvement in a local event, 3 pts for use in a famous assassination, etc.), number of generations held by the same family (1 pt per generation), monetary value (1 pt per digit in the price tag...$1 = 1pt, $1,000,000 = 7pts).
I'd recommend using at least two of the different tables, adding together the factors, then dividing by a suitable figure to get a scale from 0 to the upper artifact limit. Because the higher point value aspects are so much harder to achieve, the rarity of items and comparitive power benefit make a fairly natural exponential curve...and this can be linked pretty nicely to the narrative.
I've got a few more ideas along these lines, but that's enough for the moment.