First of all ...... wow, that's really cheap for a game. I wouldn't even reply one email for that price, let alone make a game.
How many hours do you intend to spend on those games, per game? Do you have to provide code assets/graphics, sound, everything, or do they provide the assets?
I would suggest raise the price if he wants the source code. At least add another $100 per game if he wants the source code. (depending on how long you are intended to spend on each game) If it boils down to a decent hourly rate I would say it's fine (for small simple games)
On a side note: (But that's just my personal view) I don't know where you're from, but if someone offered me $20 for a game I would tell him/her to go f*** himself, and It's an insult to anyone working in the industry doing a game cheap, and expecting source code.
EDIT: Also depends if you're selling exclusive rights to the game, or if you still have the right to sell the same game to someone else?