It seems that you're a total newbie in case of the app development if you want to find a cheap company. The app development is always an expensive thing, especially in case of difficult and high-quality projects so prepare your budget for this. And yeah, it's possible to find the cheap companies too but such low prices usually guarantee a low-quality work too.
Also while choosing the one I recommend you to pay attention on such things:
- reviews about certain companies (you can even contact their previous clients for getting real feedback)
- if you have compatible work and communication styles (for example according to popular business strategy it's better to choose a business partner with the company which has the same size as yours)
- what portfolio has the company and how good are their other work samples
- how flexible can be the certain company in negotiations
- how many specialists provides the company to choose from
As the best ones according to many reviews are considered for example Big Nerd Ranch,
The Appsolutions, Copper Mobile, Pixelmate, Appventurez, Mentalstack, NMG Tecnnologies and Wolfpack Digital.