This isn’t fully answering your question but I’m doing the same for my singled glazed sashes.
For a bay approx 1.5m high by 2.5 wide made of 4 sectIons is £1200
And a single window to side of it 1.5m high by 1m wide is £400
All balanced vertical sliders with 6.8mm branded acoustic glass. Sliding or fixed seem to be 20% cheaper. Standard 4mm glass is another 20% cheaper again.
This is from a DIY firm which is very popular online with a online calculator quote system. ( don’t think you can name companies on here? )
All my local glaziers want 50% more for the same spec!
All the secondary glazing comes from 2 manufactures (excluding frameless ones i’ve seen) and firms put their mark up on top. I’m not paying 50% more for someone to install it!
From all the research I’ve done. The expected sound reduction for windows are.
Single glazed 20db - I can confirm this is correct!
Slim line double glazing 25db. Can also confirm this!
Standard dbl glazing 30ish dB
Acoustic glazing 40ish db
Airport spec with good frames 45db
Secondary glazing 40db - 50db depending on air gap when used on single glazing. Dbl glazing in front won’t help that much more as you’re limited by the brick work of your house which is usually only 55db reduction on standard constructions.
Glass spec doesn’t matter too much as the air gap is the most important. Acoustic laminated glass does help reduce lower frequency sounds more than regular glass. But from what I’m told you need 100mm to 200mm for best results.
Frameless secondary glazing is dbl the price of these framed units. But I’ve never seen any reviews of it, but looks great using stainless glass fixings.