It depends on the type of gambling sites you are planning to open. Dice sites relatively cheap top open but they are saturated and it would be difficult to compete with traditional dice sites like Primedice unless you have money to burn to attract customers from other sites. If you are planning to open up casino gambling or sportsbook, you need ten of millions of dollars to acquire license and lots of capital reserved for withdrawal.
Yes, it will require a lot of money to make or have a gambling website, after all there have been many gambling websites that have attracted many people well. I agree with what you say about burning lots of money to attract more people and even attract customers from other websites. It requires a lot of money for large-scale promotions and also with a long time should also be.