Place an advertisement for the same project on a dedicated site such as Freelancers.net or RentACoder.com, and ask for the members to suggest costs involved as if you were the client.
Select bid(s) from those with suitable references (and past examples of similar work), double the rate(s) charged, and submit to your client. Having more than one cost option, depending on the variations you have listed above will help secure the contract.
If successful with your tender, sub-contract the work to the party you have sourced & project manage between the two sides of the contract.
Ensure that both sides of the equation sign a contract to describe explicitly what is expected by each, by when, and for how much.
Consult legal advice if necessary.
Invest the profit wisely.
Repeat.
BFN,
fp.