So, he essentially paid you $5.50 that you can't spend, and might not even smoke. FYL.

It's a metaphor.

I would have discussed the payment before... And at the same time I can't help but feel sorry for you

Don't give it to him.

As an artist, I have to say, this is why you set prices BEFORE doing the work. Usually based around the amount of effort put into the piece (sketches, lined works, and colored works are most often the basis around pricing in my experience). And ALWAYS discuss prices BEFORE doing the work. "Oh I can't afford that." "Then I can always offer you a sketch for (insert lower price here)." "I can't afford that either." "Then you should probably find someone else." NEVER let someone tell you "It's just art, I don't HAVE to pay you, publicity is enough.". People don't seem to acknowledge the time and effort put into the pieces that artists make and feel entitled to it just because THEY don't think it's a big deal. If it's 'just art' then I suppose it's not so important that they absolutely need it drawn for them, do they? Also, try advertising on the internet, it's a rather big industry if you can manage to put up with the shenanigans on Tumblr and deviantART.

You're supposed to discuss payment before you do the piece

That really sucks, but this is why you need to discuss the terms beforehand. Verbal or written contract, though I recommend a written one.

Sorry but you tell the price before you do the work.