I'm a heavy VATS user and I have not done much gun modding cause of fear I'll mess up a Gun's AP cost?

Is there a list anywhere of how a mod affects AP?

No. That, or changes to it are not listed anywhere.

I know this isn't the exact answer to your question but when you mod something you keep the old mods. So it should be easy to experiment and if it messes up the AP cost, just stick back on the old ones. Test it with common stuff like pipe guns and 10mm pistols