Effectiveness of the FM transmitters varies enormously by area. They only have a limited range of available frequencies. When we lived just outside the M25 I couldn't get mine to work because every frequency was full - mainly with inane "pirate" stations broadcasting in what sounded like English but was unintelligible.
Look at iPodworld for a selection, but I think under �10 is very optimistic for anything that works and is legal.
Look for Dension. Their stuff is much much better and works without interference, plus charges theiPopd at the same time. You may need it fitted by a pro.