Well I guess they can charge what they want unfortunately - you don't need to have scans, they are really an option, particularly if you have no reason to suspect problems. So I suppose they see it as an 'optional extra'.

Also - the reason they do scans at that 20-ish week time, is so that you have the option to terminate before the 24 week cutoff. If you wouldn't consider a termination at this point, it is generally better to wait a few more weeks, at least, if you really want to have a scan. I hear of so many people having to go back again, after inconclusive 18 - 20 week scans!