Hi, the game Greddy Cave is very detailed game, do you need as much detailed as that one, if yes then your 600$ budget is too low for such game.
and to your question, iAds and admob can be added together but apple is ending the iAds soon, so its not recommended to add iAds as the support and earnings from them are ending in possibly june or somewhere around.
And do you have the desings and assets for the game or they need to be built too?
i am a professional game developer, kindly pay a visit to my profile, let me know if interested so we can discuss about the requirements and budget in more details, and i can provide the samples as well