If this is more of a token gesture, then I would keep it simple.
Approach her with the amount of work you will need to finish the game,
decide how many hours work it will take for 100% of all the artwork required to be completed, add this up at $25.00 an hour, for the total to be payed...
Ask yourself is that reasonable value...
If so, pay in segments on work completed ie full tileset 30%,full characters and animations 50% ,full menus and GUI 10%...that way you will only pay when you receive and the motivation is still there for her to complete.
Stick to it and don't get soft, there is nothing like people to mess up your day... <img src="smileys/smiley2.gif" border="0" align="middle" />