I have looked into replacing one myself - my house is all original wooden sash bar one window which is wooden double glazed. And the 117 year old sashes are fine, the replacement bust and horrible - lesson in there somewhere.
Cheapest price I found for a 3ft wide by 6ft deep was £1,100, other prices were up to £2,000. FOR ONE WINDOW !!
My father in law is going to build one for us now.
You really ought to restore yours, I have done some of mine and they come up great if you invest a little time into it. I assume the noise and draft factor worries you, in which case you could find a company who will restore and maybe fit better glass. Although to be honest, it really is something you could do yourself.
I am a very very novice DIY'er, I just googled and did research and got tips off here. And the results have been good. And saved me thousands of pounds and kept all the character in my house too.
Honestly, think about it - I consider myself a sash restoration fanatic now, they are part of our history!!