How much is trade in value for a power mac g5?

We are not Apple but end users like you; we cannot say what Apple will pay. However, we do know that Apple gives a "no value" rating to much newer computers than yours. They will recycle it as no cost to you, however.

IF you want to do the exercise of seeing this for yourself, start here: Apple Trade In - Apple

There used to be a Google Group from Low-End-Mac Swap, now I think they are on FaceBook, that is a group for buy-sell-trade older Mac stuff...

https://www.facebook.com/groups/lemswapusa

If Apple won't give any trade in credit, maybe you could find someone who is looking for that particular model and willing to pay or take it off your hands? I have not used that group in a long time, but it was useful to find very Mac-specific things that are hard to find anywhere else.