The fixed rear side windows and rear door window seals on my disco are cracked and split at the corners.

Has anyone out there ever replaced these and if so how much did it cost?

Is it a DIY job or did you go to a main/franchise dealer or an autoglass type firm.

Any info would be appreciated.

that shouldn't be too hard to do, two man job really, for the door windows you'll need to remove the window frame from the door.

For the rear side windows, cut the rubber out with a stanley knife or from the inside, carefully prise the rubber over the metal lip to get the window out. To put it back, put string in the rubber where it fits in the lip on the car, then offer the window up to the car, get someone to hold it in place then pull the string from the inside to pull the rubber over the metal lip, I think that makes sense lol

Yeah, fairly simple job really as clutchdust says. Use a little bit of brake pipe to run the string through as you put it into the window seal (just drag the brake pipe around the seal with the string flowing through it and it will leave it in the place you want) if you see what I mean.
I agree it's a two man job. One of those very satisfying jobs when you've done it though. Good luck.

I repaired mine with a tube of black silicone sealant. Smoothed it with a wet finger before it dried and it looks fine. Much cheaper than mucking around replacing them and probably less likely to leak afterwards!!