Depends on how squashed the bubble is, whether any cracks, etc. I would think the days of £275 for a sealed Yak are long gone, though.
I would think that if the card is very nice and the bubble pancaked you'd be around the £400-£450 mark. If the bubble is just standard trilogo dings and dents then you're up to £600 and then you'll be lucky although they can be found.
The bubble was the best bit on this one and it achieved £230:
http://www.ebay.co.uk/itm/Vintage-1983-Star-Wars-Action-Figure-Yak-Face-Carded-Ultra-Rare-/252236400279?hash=item3aba760697%3Ag%3AAX8AAOSwFqJWiSM-&nma=true&si=SquXJvHoHvKJg4u0zT5HgJ0%252Flc4%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
This one went to a bloke who intends to keep him just as he is and not u-grade him, which is great. When you consider it's not unusual for mint loose examples to sell for £170+ I don't think he paid too much. At least he has an affordable carded example
