A LOT. I don't mean just by development as well, development should require INSANE amount of man hour put into it for it to be different from others and make you entertaining enough so that gamblers would prefer you over others. Than you will need to spend money on security which will be really expensive. Than you will need to spend money on marketing.

This is one of the highest ones. Almost all websites have signature campaigns nowadays, from casinos to altcoins to tokens and so forth. You will have to spend so much money that I can't even begin to talk about it but even with just these, you can see that its HUGE.