Hi, marcosmartinez...
I will assume that all iPoker skins have the same rake structure -- that may not be the case.
This is the William Hill rake chart:
http://poker.williamhill.com/help/rake-limits-ipoker/
The rake structure below applies to all USD/GBP/EUR tables. For cash tables, rake taken is based on iPoker's rake ranges which is the amount of rake generated per flopped hand (we operate a 'No flop, no drop' policy which means that if your hand ends before the flop, we will not charge any rake) and depends on the cash table limit you are playing on.
For NL10
players Rake Max
2 0.01 for each 0.15 in the Pot 1
3-10 0.01 for each 0.15 in the Pot 2
This is not the whole story though.
First off, you only "pay" the rake if you win the pot, but for bonus allocation purposes, the rake paid is credited to each player in different ways, depending on the room, and I can't find how iPoker allocates the rake.
I *think* though that they credit you with rake in proportion to how much you put in the pot.
For example, if game is 6-handed, and the pot is 6 ($ or € or £) then the rake will be 0.40
If three players entered the pot, in proportion, 1 , 2 and 3 then they will get credited with 0.07, 0.13 and 0.20 cents of rake.
To convert this to BB/100 is nearly impossible, because it depends largely on your playing style.
Nits enter few hands, and therefore play lots of hands where they get credited with no rake at all. Loose aggro players who play a lot of pots, and put lots of money in them get credited with large amounts of rake.
For your own particular situation:
300 € = 3000 BB
so that is 12 BB/100
For comparison, here is the PokerStars rake chart:
http://www.pokerstars.com/poker/room/rake/
According to that chart, PokerStars charges 4.5% rake to a maximum of 1.25 €
1 in 15 works out to 6.66% with a larger cap.
So that is the number that truly compares William Hill to PokerStars.
Other factors to consider:
How big is your edge on each site?
Will your playing style be the same on each site?
Best regards,
--VS