Brush mate... dont sell yourself short here bloke!!
Its one thing for doing work for a friend but its another is he is willing to pay for a custom job.
I guess income plays a big role in friendship pricing though if you ask me... if one wants a custom job which requires you to to sacrifice your time and effort away from family and other issues means there is compensation to be dealt with.
All this set aside though... A friend of mine while i was living in Illinois asked me to do a custom portrait of two girls for him so he said he would pay me well, but when i told him my price (still reasonable for the size 48"X39"=$500 time commitment for my effort) he thought i was some sort of crazy, looking for a free feed or something...not cool!!
I have to say that you really need to be sure in backing up your artwork for a confident sale. Im sure there are many pros here that sell work everyday to consumers with alot of rejection in body language but ya need to look passed that. My problem was i was too flustered about his comment on my pricing so i deliberately sabotaged my artpiece (out of pride for myself) just so i could convince him it was impossible for me to fix.
This friend of mine (no longer a friend because of this incident mind you) has not been the same since in terms of closeness.
Its really a sad story kinda... we were very close and shared much personal history with each other until this event.
Long story short... if you feel he can pay for it and is confident in still being a good friend then i say bill him a decent friend price otherwise just get your material costs with a little bonus and hope he dont go crazyman on you.
Good luck brush