Depends on area you are working in mate. If you are doing subby work for some contractors I know they would be looking at £350 max and that includes materials. Some might pay you £400 most subby work is £15m2 -£25m2 for normal tiling labour only again depending on area might be more.
Way the work sittuation is better going in sensible rather than greedy and you might pick up more work from contractor.
I would probably aim at tiling three kitchens in first week tile all three first and then grout all three at end of week. You will soon rattle through them and if you aim at three you can then bill for say £1050.00 or whatever price you agree.
If you do get job and are supplying materials make sure you divide the labour and the materials because if you dont you will be taxed the 20% on your materials. Some guys I know have very expensive materials
Highlander