hmexx, to be clear: you pay $5000 to the developer to seed full time development for a month or two and on completion you will in addition spend $3000 promoting the product.
An open question is: how active of a partner would you be once the business (hopefully) becomes a real/profitable business? Are you offering to be a full active partner in running the successful businesses that you seed with this offer? If so, then your offer sounds like a good deal for developers.
As the offer is written, the $3000 could be spent on AdWords and similar services.