As has been stated, Air Southwest operates on that route:
http://www.airsouthwe...k&utm_campaign=nqyweb
(It's the ONLY airline on that route, so it's pointless looking for air fares anywhere than on that site).
Standard fares, with Air Southwest, are usually £59 or £76, depending upon the day of travel. However that assumes booking well in advance. You need to add on a card handling fee of £1 per flight if booking with a debit card, or £4 per flight if using a credit card. Additionally you have to pay a £5 'Airport Development Fee' when departing from Newquay (but not from Glasgow).
Dzug2 has provided the off-peak 'walk up' return fare of £207.70. The 'anytime' fare is £404.00 but it seems that the off-peak fare covers most (and possibly all) trains anyway. The availability of discounted advance fares seems to be extremely limited but, on a random sample date I looked at on the National Rail website, it was possible to pay £94.00 outbound and £89.00 return, making £183.00 in total. (There are actually cheaper fares but they involve getting stuck at a London mainline station overnight).
http://www.nationalrail.co.uk/
As Dzug2 indicates, it might be possible to pay less by buying separate tickets between Glasgow & London and between London & Newquay but, on the random date I selected, I couldn't do so.
You can get from Glasgow to London for £9 each way (or sometimes for only £5) if you book early on Megabus:
18:57 Mon 31st Jan 2011