If you have skills to make a gambling site, $300 can get you a domain and server.
The bankroll is something you can get investors for after they are impressed with your work. To do this by yourself can take many months of hard work. Or, you can get paid by the hour working as a developer and build a gambling site for someone with bigger funds.

That is of course if you have the skills to start a gambling site.
But, there is a saying you cannot do it if you have to ask.