I'm looking to refurb some sash windows and replace the agine glass with double glazing panes. What sort of ball park price can I expect to pay for a double glazed pane of approx 16" x 20" if I'm ordering at least 4
Big-all is right. You'll probablyly need completely new sashes to get a rebate big enough for a D/G unit and even then I doubt that you'll get more than a 8mm gap (16mm total thickness of unit).
You should certainly shop around for prices, though. When I needed some units a couple of years ago a supplier in middle-class Surbiton wanted three and a half times as much as the glass factory I eventually found on an industrial estate in Hackbridge. Suppliers usually base their prices on area + an element of fixed costs. The good ones can give you a quote over the phone.