I think it needs a lot of investment if you are planning to start making your own bitcoin gambling business i think 30-50 btc is enough for start a sports betting site.. paying for a developer or buying own gambling script it can costly around 0.1 to 1 btc or more depends in the design and how many games inside the script.. also you can't manage it by 1 person you need to build a group. You should have support to maintaining your site active, developers and i think you need to pay also for beta test of the site just to trace some bugs to prevent frauds in your site..