You are giving far too little information as to the actual specs of the job for anyone to give you sound advice.
Altho, in general, your idea of a flat fee plus costs plus a percentage is a good one.
The problem is that it sounds as tho you may be dealing with inexperienced capital, and since it is harder than ever now to be successful with a paysite system, you are likely to have dissapointed clients when they realize that there is more to this game than putting up a paysite and raking in the money.
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