To build your own ATS you’ll need at least 1 front end developer, 0,5 business analyst, 1 tester + 0,5 tester for 2 months, 0.25 devops specialist, 2 backend developers, and 0,5 project managers roughly for a year (this will obviously depend on your project complexity). While I can’t be sure about hour rates for such a team as different agencies charge differently, I do know that you can find the best cost-quality ratio by outsourcing your development to Eastern Europe.