Went into Blockbuster yesterday and they had in a few decent offers on this week. Best one by far was COD: MW2 which was down to only £14.97 and they had copies on both PS3 & Xbox 360 formats available. Just found out that this is also available online (pre-owned) for £14.94...
Not sure if instore versions are pre-owned (didn't see any at full price and no mention of pre-owned on it). Either way I think it's still worth £14.97 IMO. Suppose it's for you all to judge for yourselves though.
Not sure how long the offer is on for.
Checked the selling price on Amazon this morning and it was still at £33. Avg. price for a used copy on eBay (sold yesterday) was about £22.