Assuming that the company is reputable and they have:
A. already accomplished multiple similar nature projects,
B. enough project management and risk management experiences,
C. proper Agile or Scrum development strategies,
D. clear reporting mechanisms for their clients (in this case for you)
E. at least some sort of PPS (post production support) to fix the first and important bugs before your lunch your product

I think anything from $25-30/hour for 2 person team can be a good hourly rate for you.

Also based on my experiences finding a decent ROR developer is much harder than for iOS/Android developers or those working with other web technologies. This applies both for North American as well as for offshore resources.

Myself I have been an inside sales manager for Armenia based software development company for over a year, worked also in India in their IT hub Bangalore for 4-5 months Currently I am doing marketing consulting for some of the biggest software development companies from North America.

I would be happy to have a call if you want to know the best sources to find your developers as well as most important aspects to look at before signing your contract with them.

Best Regards,
Artash