Why do you want to put carpet tiles down in a kitchen? Won't they get really grubby and oily, plus you wont be able to clean them easily.
Maybe you can try Linoleum tiles which can be easily mopped or just plain lino. These might be cheaper than using carpet tiles. Also Today's lino looks alot better than 20-30 years ago.
A room of 9 feet by 21 feet, equates to 18sq metres. B&Q sell individual 50x50cm carpet tiles, at about �2.98 each, so to cover 1 sq metre, this would cost �5.96. This means to cover your 18sq metre kitchen, you would need to spend aproximately �107.28. This would obviously exclude any adhesive you may use, plus you might want to purchase extra tiles, incase you need to replaced damaged ones in future.