Its probably not signifigant for the casual player. What happens is, in order to get players to play at their site, specifically, high volume players, many sites offer "rakeback", which is a % back of all the rake you pay. Some sites don't offically support it (such as Party Poker), but you can usually find an affiliate who is willing to give you some of your rake back in exchange for singing up with them. Some sites flat out don't offer it (Poker Stars) and others will use rakeback as their main lure (Absolute poker). For example, last month, I played around 50 hrs, I made 750, which is roughly $15/hr. I moved up limits this month, so I expect to make more/hr. I get this playing 4 tables at a time and at shorthanded tables, which mean more hands/hr, which mean more rake paid.