Restoring old windows?

My new double glazed wooden framed sash windows are £1600 plus vat per (quite large) window. House built in 1904. The cost includes fitting and priming but the top coat of paint is extra.

We've been doing ours gradually over the past 15 years, starting with the original single glazed units and latterly replacing poor quality uPVC done by the previous owners. The wooden ones from about 15 years ago are lasting well and haven't even been painted in that time.

Compared with the original single glazed, obviously the thermal performance is far better. Too early to compare warmth with the uPVC, but the frames are narrower, more light comes in and they look vastly better.

So I'm happy, but doing them all at once would have been mighty painful.

Get thyself over to Buildhub - a community owned forum where I am on the moderation team ("Ferdinand") which is about self-build and renovation.

We've been going since about 2016.

https://forum.buildhub.org.uk/

I did the four windows on my Victorian flat. I replaced the parting beads (between the panes) and the staff moulds with new timber ones that have a slot for a felted draft seal. It wasn't particularly difficult, I'd say a couple of hours per window. The result is much more air tightness.
Secondary glazing is a better option than double glazing for historic sashes.
Don't even think about uPVC windows if you care for the planet and its future.

I'm restoring mine, I never did anything like this before, skill levels building as I go. It'll take years to get them all finished. The ones I've done are a lot less drafty. They'll never be as draft free as modern windows but m wife likes the look of them so we're not getting new windows.

While original sash windows are nice they make it very hard to keep the house warm, we eventually went for upvc double glazed sash window replacements and the cost was quite reasonable although we only had 6 to do, not 30!

Just spray a load of builder's sealant around the windows to eliminate drafts

If the sashes are original (single glazed?) then it's likely any company quoting will be quoting for new, sympathetic, double-glazed sashes. It's not cheap. And unless you're in some sort of similar industry, it's not something I would DIY.