You aren't trying hard enough.
Persistent Wraithstrike can probably be carried off by someone much earlier with only minimal use of splatbooks. A few of the substitue-something-for-metamagics do carry a limitation that you have to be able to cast the metamagic'd spell already (thus would retain the 15th level requirement) but many do not and can be used much much earlier. The one that uses your bard songs, for instance, I recall carries the restriction.
Clerics can cast persistent Divine Power at 7th, easily, using Divine Metamagic. An Ultimate Magus could persist Wraithstrike around 10th w/o much work or wonkiness. There are a few PRCs that allow the player to simply apply a given metamagic feat w/o paying an adjustment a few times a day (Wyrm Wizard is one example).
Aspenor's point is that divine power on an item is hardly game breaking and out of line with what others can do already.
Which is more game breaking? Your wizard crafting a Divine Power clicky, or your Cleric having Divine Power persisted 24x7? In those environments when players use the feats, PRCs, etc. to their strongest, that Divine Power clicky isn't that expensive. In fact, the command-word activation (the opportunity cost) becomes the real cost, far more than any gold. It takes some sort of action (even swift or free) and in a really intense, highly optimized game those are your real limiting factors. It isn't money or spell slots; it's action efficiency.
Now, if you're running a restricted game, you're not likely to allow many of the splatbooks, etc. You're imposing your own rules to a restricted set of content. That's great, and perfectly fine. In that environment a Divine Power clicky may be "too powerful". You may not allow Divine Metamagic ... it is in a splat book and afterall, an optional rule.
Recognize though that is YOUR RESTRICTION on the materials that brings it out of line, and not something that is mandated by the publisher of the game or hard in the rules. If you cast your eye on the breadth of it, a divine power clickie using the basic rules is NOT out of line.