if your website is even remotely related to casinos (casino, poker, gambling, etc) then you can - and should - accept guest posts because.... well, because free content
As for the pricing aspect, I have no idea. I have wondered this myself, too but I haven't found a definitive answer. What I can tell you, though is that it has A LOT to do with the stats of your site: If your site has good metrics (DA / DR / TF, lots of authoritative and / or relevant RDs, lots of organic and / or referral traffic, a small number of OBLs, etc) you should ask for $100-$200 for a single guest post.
If, on the other hand, your site doesn't even get traffic (no matter if all other metrics are good)... I don't think anyone will pay you more than $20-30 to be honest. Traffic is the real deal nowadays because google has put monopoly on it and it's very hard to get traffic consistently for the average Joe, so those who can do this have a valuable asset at their disposal. Actually, if I think better, the amount of organic and referral traffic your site gets is probably the biggest influencing factor in pricing a guest post accurately.