forneamax - myhammer charge £80 every 3 months. When you win a bid you pay a percentage of what you bid - on a £1000 job this will be about £60 + VAT. It is very difficult to make a profit on this site unless you either work at a loss, do shoddy work, or work on a cash only basis using inferior materials. Oh and beware of signing up - myhammer tie you into a 12 month contract - so they guarantee £240 from you even if you never win a job!
Not sure about mybuilder - have not looked into that.
The cheapest internet job site is ratedpeople - they charge £40 + VAT per quarter, and a fee for every job you bid for (even the ones you dont get - you pay to buy the customer contact details). On a job the customer values at £1000 - the fee will be £18 + VAT.
The main advantage with internet sites is that you can go for jobs that you think are liklely to be profitable, achievable, and fit in with your workload. I have tried a few established trade directories - e.g. Yell - charging me £60 a month and I got just 1 job in 3 months! On balance, if you are going for internet trade I have felt that ratedpeople was the best for building up tiling business - I have used them for a year, and I am now getting referals of new customers from satisfied previous customers from this site. Hopefully another 12 months of this and I wont need to pay ratedpeople!
Lastly - I appreciate some excellent advice about trade contacts - but I found myself in direct competition with several very established tilers - so I stood back from that to establish my own name, and work to build my reputation up.