The following graph shows comparitive rake at the major poker sites compiled from shorthanded fixed limit data at marketrake.com:
There are a few interesting things to note about this graph. Firstly, 2/4 seems to be a bit of a sweet spot in terms of rake - cheaper than 3/6 and even 5/10 at some sites.
Secondly, the rake is very different from site to site. Cheapest by far are Full Tilt, Pokerstars and Everest. Sites like OnGame, iPoker and Micro sit nearly 50% higher. This puts things like VIP bonuses and rakeback in context. For example, you probably pay more rake at Microgaming compared to Pokerstars even if you have 30% rakeback.
It is also interesting to compare NL with limit. NL is typically cheaper than Limit, but the difference is not really marked until you go above 100NL
Lastly: never, ever, ever play 0.5/1 at Pacific!