You need to talk to an organisation who specialises in this sort of job - there's plenty about because lots of people do this.
Every job is unique because shapes and structures are so different, so it genuinely is difficult to give a better ball-park than "It'll be �15k - �50k". There's a factor of three in that figure so you see the difficulty.
The main difficulties in general are:
Where does the staircase go and can I get the headheight required under Building Regs
How much strengthening of the ceiling joists is required
How am I going to satisfy the fire regulations requirements now its a three storeys - this normally needs fire doors being fitted to every access that leads off the main exit staircase on the ground and first floors. More expense.
How am I going to satisfy the ever-increasing thermal requirements of Building Regs by insulation between the plasterboard of the new room and the roof structure above.
How am I going to get light in there - Velux or new dormer window.
Unless this Victorian terrace is in an expensive part of town or an expensive part of the country, it may be more cost-effective to move to get more space.