In my opinion, all casinos that charge additional withdrawal fees on top of the standard network transaction fee are just typical scammers.
Many casinos (just like cryptocurrency exchanges) will force you to overpay on the withdrawal fees, but they are not going to tell you what is used for the mining fees and what is their own profit.
You can put them in three categories basically:
- Free withdrawals with no fees at all.
- Dynamic fees that change according to network conditions.
- Fixed fees that are usually way to high and are only occasionally changed by the site owners.