Lizzie,
Both types will give you protection (providing they are fitted correctly). Some of the custom made ones can be a bit hard when the helmet is pressing on your ears which can get uncomfortable for long periods of time.
Also not all harmful noise passes down the ear canal, noise can also be generated from the helmet shell to the bones behind the ear.
For anyone who has custom made plugs, remember they do not last for ever, as the shape of the ear changes over time, so the plugs should really be replaced every 5 years.
One things for sure if you do not wear hearing protection when riding you will get noise induced hearing loss and perhaps Tinnitus (not a very nice thing to have).
As for price, usually upwards of £50 for custom made ones and about £1 a pair for disposable ones.
Google search for suppliers as there are loads out there.
Why don't helmet manufacturers invest in designing quieter helmets, it ain't rocket science
[Edited on 27-10-2005 by Dementor]