Seconding this, the only reasonable way to do this business is either fixed money or fixed output, you can't have both at the same time. You can give estimates,

Seconding this, the only reasonable way to do this business is either fixed money or fixed output, you can't have both at the same time. You can give estimates, yes, but be very clear that this is an estimate, not a binding offer. If you're selling fixed money and fixed output at the same time you're always bidding against yourself.