We just had a l-shaped dormer in our Victorian terrace and paid £55,000 (Greater London).
Here's a few tips/comments;
We used a company who specialise in lofts and they literally did everything, from survey to final building control sign off. Possibly could have got it a little bit cheaper, but we just wanted someone who could come in and do it all.
We paid a flat fee and it was built to a set formula eg you get a certain amount of plug sockets, lights, windows etc. we could have paid more for customisation, but we were happy with the set formula.
Get a bathroom up there if you can. We went for a separate bathroom rather than an in suite, so it could be used by the whole family (we only had one bathroom and loo, so it's really useful to have another one). Our company fitted the bathroom, but we had to pay for the suite.
You may need party wall agreements with your neighbours, get these done in good time before your works start.
We have a window put in over the stairs, which brings light into our hallway and I'd recommend getting one if you can (ours was included).
Our company built a cupboard in the dormer and fitted lights in there, which are handy. They also put in two doors so we can access the cupboard from both sides, which makes access easier.
Don't forget that you will need decorating and flooring. We decorated ourselves and spent about £200 on paint and decorating equipment. You'll also need flooring, plus cost of decorating/tiling the bathroom.
We looked on Rightmove for similar houses with loft conversions, so we could get an idea of size of the space and layout. We got some good ideas of what worked well and what didn't.