I've told my players I'm going to allow them to stick piecemeal additions onto an item to make it a better power item. Here are the simple rules I came up with for cost.
Improving Power Items - You can pay to improve a Power Item by having it gilded, gems mounted, improvements put on it, etc. The cost is 150% of the improvement in the value of the final item. If you have materials on hand (say, you have a $1000 gem), the cost is 50% of the added value. For example, you have a $1000 gem in hand and want to mount it on your Power Item staff. You pay $500 and the gem is mounted, improving the value of the item by $1000. If you lacked the gem, you could simply pay $1500 and have the item improved by $1000 in value.
Simple although really expensive. It makes adding on possible, but finding stuff to add on in more attractive, and finding a bigger item as-is is the best.
How has this worked in actual play?
It hasn't, yet, as it has only been on the books since December and we haven't played much in the interim. Needless to say, I don't think the wizard players in my group like this. It's kind of unreasonable that it's a sliding scale - that the value of the improvement determines the cost of making it (it's much more expensive to mount an emerald than an agate, or gold than silver.)
But on the other hand, money is directly turning into power.
Further, this is basically allowing piecemeal improvements. Add another piece of bling to your staff or amulet or whatever, and it's more powerful. You don't need to save up to go from Ornate +2 to Ornate +3, or worry about getting rid of this $12,000 power item and getting a $12,001 one to hit the breakpoint and get more power. You can simply add a tiny bit more to it.
I figured there were three ways I could rule:
No Improvement - sorry, you need to entirely replace the item.
Improvement, with a high cost - you can improve it, but with a steep markup. This is the above.
Improvement, with a low cost - you can improve it, with a nominal cost.
With one option to modify it:
Improvement By Level Only - you have to do it in a big jump, either by adding a prefix (Ornate, Fine, Balanced, etc.) or by making a full jump from a specific Power Item level to another.
I feel like the 50% approach is the middle road - it's still generally better to find a new item and keep it instead of improving your current one. But you can mill your coins into power. It's still pretty unrealistic - modifying jewelry can be a fraction of the cost of the item or more than the item is worth; modifying something so it's more valuable and not just a bling-studded piece of gaudy crap (which could reduce its value by making it less artistically valuable) isn't trivial. But it's DF and I'd like simple but costly.
Originally, I was thinking 100% of the cost if you lacked the item ($2000 to add $1000 to the value of the item) and 50% if you had it ($1500 to add a $1000 gem, say, to the item). But I realized that encourage the odd behavior of buying gems with your cash and and then bringing them to get mounted. So I went with the cheaper cost. You can still walk into the dungeon and score a sweet $15,000 pile of loot and turn it into $10,000 worth of improvements on your power item. Or just buy a $15,000 power item, if that's better.
So that's how I'm giving this a try. We'll see if people bite, or declare it's too much and just replace items entirely. If so, that's really okay with me - this is just giving wizards an option to turn money into power without sacrificing the sunk cost of the item they already invested in. I have some ideas on improving existing weapons, too, but I'll save them for when I have time to fully write them out.