In any case, you should completely design your game ahead of time. Development is expensive, I won't lie. I wouldn't even consider working for less than $25 / hour (or, since I can spend about 20 hours per week doing a project of some sort, $500 / week).
I don't do "per project" quotes right now unless it's extremely in my favor. I've been burned by that too many times. Seriously, there were a couple of projects that once all was done I was earning about $2.50 / hour for the time I put in! lol
So, you can see how quickly costs can add up. If you have the whole system worked out ahead of time, it will minimize your development work and, thus, your total cost.
Hope that helps.