It's hard with 50$ honestly, SBET is only available on coinstore so you need to deposit, withdraw witch has some fees and you also need to have an EOS account which still is quite a hassle. Betfury can be bought on BNB chain but it needs to deposited in your betfury account. When paid out in BTC/ETH/USDT/TRX/BNB there also will be a withdraw fee on each of them, When paid out in BFG it compounds and there is only 1 fee if you end up withdrawing. Rollbit is impossible to invest in as NFTs are at least 700$+. Openbet is probably the best option but also the highest risk/reward. It's easy to get funds Arbitrum network and right now just holding OPENBET will give the monthly dividends dropped in your account without the need to do anything.

So for 50$ I would personally go for Openbet which for me also has the most upside potential (and risk)

I would say from 300$ or up you can easily pick whichever one you want without overly having to worry about fees. It's also one of those things, getting things in gear buying your first one is the hardest but once you see earnings flow it the hard part is to stop buying more :)

Good Luck!