I have just had mine done and it was 20k all in including all regs etc, I am very happy with it and before it was done I thought I could have maybe tackled it, well there is no way I could have done what they did, its much harder than it looks, replacing floor joists steels etc.
We were lucky and didn't lose any room as we had the stairs put in the space of a walk in wardrobe and they now come off the hallway.
It has made a real difference to the house and would recommend it though