What does it actually take and need to start out a new sportsbook/casino/dice site?
Guts. Dedication. Seriousness. Big Bankroll.
What amounts needed to start up, what providers for themes/scripts all that stuff etc?
Scripts, building website, design and layouts, developers, domains are somehow can considered as cheap or not expensive. You need to put big money on advertisement
(you can choose to lowered the budget for this but today with lots of famous gambling sites, you need to match the competition so a big money is needed here) and bankroll as you must have the ability to pay all your users. Just set a maximum bet for it.