I had 2 quotes from specialist company's the first to do the complete refurbishment of the stairs £16500 the second to design and make the steel work only £5500.

With them sorts of quotes yes I would agree with your joiner friend, however I decided to install and do the maximum work I could and only use specialist when required. I contacted the architect that had already designed an extension I done on the house. Sent him the rough drawings of what I wanted he did all the calculations of the structure and building regs.

Then I contacted a local fabrication company to make the steel work, once completed I removed the old staircase, and with the help of 3 others installed the steel work. Electrical work had been completed. The oak treads I bought as 12 solid oak pieces cut to size, then I completed the cut outs for the steel plates and sanding. Plastering of all walls by a plastering company. The most expensive part was the glass, and fitting by a glass company. Painting again me, handrail bought and fitted by me

The project actual costs were
Design drawings £450
Steel work £1200
Plastering £675
Oak treads £750
Lighting £260
Glass £2350
Labour free
Bottle of wine for local framer who off loaded the 350kg steel work from truck

Does it add value to my house not sure but we love it!