How much is my iPhone 5 worth if the power button doesn't work?

It's worth whatever another buyer is willing to pay for it.

As a rough estimate? Figure that a 'new' iPhone 5 would sell for about $450-500, and it costs $269 for an out-of-warranty service. I wouldn't pay more than $150 for an iPhone 5 with a broken Sleep/Wake Button, personally.

But I'm not in the market for such a device.

Try either of these and get an estimate:

http://www.gazelle.com/

https://www.apple.com/recycling/

(I can tell you that gazelle offers me $265.00 for my 32GB iPhone 5, in "flawless" condition and if I get it unlocked by AT&T before I ship it to them).