How are they allowed to get away with this? And is there anything we can do stop it?
I'm talking about the really big players here, not some tinpot back-bedroom registration firms.
The more specific a claim, the more likely it is that an advertiser can get called to prove it. Deliberate misrepresentation would result in at least a settlement to stop making the claim and to make no similar claim in the future without adequate proof, or occasionally stiffer penalties. "The lowest domain registration price in the nation" is fairly specific. Of course, firms can still weasel around this issue if under some circumstances (e.g., you register a thousand domains) it is true. Such details should properly be noted in the ad.
If you are in the UK, you should contact the Advertising Standards Bureau if you think a company is making false or misleading claims.