depends on the quality of felt, method of laying and wether any roof boards need replacing which is often the case after a roof has leaked.
i paid �1,700 (inc vat) for my garage although its approx 40sq metres and all the roof boards were replaced took 3 days and they had to get a skip in.
I usually do most things myself but the area was just to large to do on my own , they also used hot tar/bitumen,which isnt easily available to a DIYer.
as chas says-it depends on whether roof decking needs replaced-if it is only a cap sheet (one layer all over existing) needed this is obviously cheaper
if drips and upstands are needed(usually so) this adds on to the price-most felt is torched on nowadays-i e there is bitumen already on the felt and this is melted by torching-still not a d i y job
would new lead flashing be needed anywhere round the edges also