the hmv trade in deals are usually specific to a particular 3 games (usually newish ones) to get a new release cheap.
red dead for £30 seems too high BUT if RDR was one of those 3 games listed for the deal with halo then thier system could have had RDR listed for as much as that to make the offer go through the till. Basically if you see one of thier posters saying get F1 2010 for £20 when u trade in (for example) RDR, fifa 10 or mw2 then all of those games trade in value will be £20 that week. the values are always fluctuating and they dont seem to take into account online prices (they gave me 20 quid for blur. i paid 18 for it etc).