Hi,

Well, the budget is something that depends on your app's complexity and features. A simple app may cost you less, but a complex app will involve more man hours.

Hiring an agency has it's own benefits. You can rely on them, they can deliver an app on time, test the app thoroughly (as they have dedicated QA teams). On the other hand, Freelancer may cost you less, but may not deliver agency benefits.

In addition to Upwork and Freelancer, you can find reliable companies on Clutch.co. The biggest reason of using this site is the company reviews are verified. They conduct telephonic interviews with clients before accepting reviews.

I referred one client to an agency named Apptunix and they were quite good.

I hope it helps. Good luck!