From an insurance companies building cost estimator. For garages (other than integral), rebuilding costs can range between £5000 for a single pre-fabricated garage to £8600 for a double pre-fabricated garage. Costs for a purpose built garage range between £11800 for a single to £16000 for a double built garage.
Before you start, however, you should consider if you or any future purchaser of the property might at a later date wish to expand over the top of the planned garage. It may be a selling point later on to be able to show that the garage had planning permission and appropriate footings/walls to cope with a bed/bathroom extension above it. The additional cost now would be miniscule compared with the potential cost of demolishing this planned garage and starting again with a full extension.
When I did mine at my previous property the cost of materials wasn't significant. You will be mainly paying for labour. If you are handy DIY persons you could manage such a job.
And "Nuisance Value", in 2010 (http://www.pistonheads.com/gassing/topic.asp?t=863822) had this to say:-
How long is a piece of string? Garage - you could spend £5k or £30k depending on what you want. You've not mentioned what sort of roof you want, i.e. flat felted or pitched with pantile or slate, though I will presume you're going for a single skin block wall.
Block prices vary depending on the following;
size - 4" or 6" (just to keep it basic we'll stick with these)
fair faced or not (for painting, or not)
strength (depending what sort of roof it's supporting)
So really you need to know what you want. Aside from all that, you could try ringing up a few local building suppliers and asking how much blocks are? I've not bought any for a couple of years now but you should allow about £1 per block for 4" and about £1.50 for 6", you're not going to be buying enough to get quantity discounts. You'll need about 50m2, or 500 blocks so anywhere between £450 to £800. Plus VAT of course, maybe delivery too.
Depending on where in Glasgow you are try Beatsons in Cumbernauldhttp://www.beatsons.co.uk/contact.php#Cumbernauld
Crispy Bacon had this to say a few years ago (http://singletrackworld.com/forum/topic/any-builders-on-here-how-much-would-it-cost-to-build-a-garage)
When I had my detatched double garage built 10 years ago it cost 17K (the garage door alone was 2K), it's 7m x 5m with a tiled pitched roof. It has a breeze block inner, a brick outer & if we insulated the cavity you could live in it 
Personally I'd suggest that you build the garage as big as you can get away with on the ground available, I'd also go for a flat roof as you may wish to, in time, build on top of the garage to make more bedrooms/nursery rather than say moving elsewhere. If this is a possibility then make sure the garage foundations put in now are able to take the extra weight of any future extention of the garage upwards.
Here is a more recent discussion (2013) with the original question by "Jan12"
(http://www.diynot.com/forums/building/cost-of-building-garage-27k.366295/)
So I think we can conclude that it is not cheap but at least if you have one built you can actually make it the dimensions you want and your car will actually fit in it.....................
