How can cashback is costly while it will be working only when players lost their money? Rakeback costs more compared to cashback because it is not based on player's losses. Even the site with huge marketing funds like the one you promote in your signature does not offer cashback. It shows clearly that cashback is not unique, but it is not what is wanted by most players. Just use simple logic, if a player want cashback or hunting for cashback, means that they prefer to lose while gambling as if they win, they wont get cashback.
I think nothing is costly in here because we ain't buying an expensive item. Gambling must willingly be done by us and it doesn't always mean that we should play with huge money. It doesn't make sense to lose much only to get a bigger rakeback or cashback as their max amount won't still exceed on the amount that you deposit.
It will be better if you can just focus on hunting big multipliers in the game, as that is the ones that can truly give you a big win. I don't think there are many people who cares about that two because again their rates are only small but if there's a type of bonus that people cares the most then that would be the monthly.