Hi Dumm,
A few things to think about.
a. The thickness of the units is important. They will be either 20mm, 24mm or 28mm thick. eg 24 mm unit has a 4mm piece of glass, 16mm spacer and 4mm glass. Too thin a unit and it will rattle when installed. Too thick and you wont get the unit in.
b. Old units may be stuck in with security tape and hard to get out unless you know what to do.
c. Old units if you damage the beading when you take it out you could have, being DIY, problems getting replacements.
d. Same with the black rubber gasket.
e. If you take the unit out you should see some packers at the bottom of the unit. Make sure they are in place when you replace the unit.
Double glazed clear units should cost about between �30 and �40 per square metre.
If you dont know what you are doing look in yellow pages or freebie press. There are usually small companies that advertise replacement of misted units or hinges.
However the main thing is the thickness of the units!
I dont want to put you off. People have changed units without any problems, but if you are not DIY inclined then you may make problems
Rgds Al
PS Hope I havent dazzled you with science, if so I apologise.
PPS I work in double glazing