A builder we use at work quoted me £10k plus for a traditional detached single garage. I think you could buy the materials and build it your self for somewhere around £2.5K. Work it out, bricks measure 102.5mm wide by 215mm long by 75mm high plus a 10mm joint. They cost around 20p each.
Concrete for a base is about £65 a cubic metre and the slab will need to be at least 150 thick.
Concrete sectional garage cost about £1500 including delivery and building (plus slab).