I had this offer come through in the voucher booklet that Costco send out every now and again. The voucher is for a £70 discount. They usually hand the booklets out as you enter the warehouse, but even if you don't have the voucher they will scan their own at the till.
Please bear in mind you will need Costco membership to purchase this.
Having had a look around it seems to be a decent price for a well specced laptop.
This price includes the VAT. It is 479.99 excluding VAT.
Dual-core Intel® Core™ i3-370M (2.4 GHz)
Genuine Windows® 7 Home Premium 64-bit
500GB SATA 5400 rpm
ATI Mobility Radeon™ HD 5470 Graphics
Video memory: 512MB
Blu-ray Disc™ ROM Drive with DVD SuperMulti
1 x HDMI (digital video/audio)
1 x VGA (analogue video)
Dolby Codec: Dolby Home Theater® v3
2 Year Warranty Included
It seems to be selling for around £660-770.