new developer

It is impossible to give a good answer to the questions, but some thoughts:

• 0 In which country do you operate ?

• 1 How well structured is your code ? How much time will developer need to design th in app purchase ? How much test do you expect ? So that could range between 20 hours and 50 hours (I don't think a developer will accept for less), at a price of 50 to 200$ / hour (tariffs are very variable, depending on the qualification. So you could plan for a fe thousands $.

-2 It all depend on the number of users you have. So impossible to say. In addition, price will of course depend on the expected yield rate of ad seeing.

At the end, take care, it is difficult and risky if you are a new developer to subcontract development and have a good control of the work (and the capability for further evolutions). Unless you know a developer you trust, I would advise not to do so but better learn how to implement in app purchase.

Note also that convincing customers to pay for ads in your app requires a very convincing business case.

Hope that helps.