If you are even moderately handy, you can rebuild the unit yourself. They are called "IGU" or "Insulated Glass Units" and they aren't that complicated.
Getting the glass out of the frame is probably the hardest part, and that just requires figuring out where the screws are in the frame - they may be hidden by goo.
Once you have the frame separated, a razor knife around the outside of the glass to cut the sealant and the spacer, and you have the glass separated. Use a razor to scrape off all the old crap - sealer, goo, separator... All of it. Now - clean, clean, clean some more... And then clean it again! There is a light blue spray can of cleaner in Home Depot called "Clear Glass" (or similar) that is designed for windshields and the like, but you can also go to any Safelite Auto Glass installer and buy the cleaner cans that they use - I'm pretty sure it's the same stuff. What you want MUST be streak-free and not leave any kind of residue on the glass.
Now that the window is clear.... How to build the IGU? The separator / desiccant strip is available from Amazon here: https://www.amazon.com/dp/B0149J3L6C...v_ov_lig_dp_it
The stuff is self adhesive, and you want to use it with the foil side to the edge of the glass. Peel one side, place it about 1/2" inside the edge of the glass, and install all the way around the window and leave a small gap at the top. Then just peel the other side and CAREFULLY line up the glass and stick them together.
Now the sealant. The original sealer is a butyl product that "never dries" yet manages to break it's own seal over time and let the humidity in. Boo on that. I used 100% GE Silicone 2, which I have NEVER had a problem of it losing adhesion! The idea of the small gap in the separator at the top of the glass is to allow you to fill the IGU with gas - more on that in a minute. Squirt the silicone into the gap between the two panes and fill that space from the separator to the edge of the glass. All the way around - BUT LEAVE THAT GAP AT THE TOP. You'll get to that later. You can see with the silicone if there are any gaps against the glass, so it will be easy to get it full and solid. Set it aside and let it cure for at least a day.
Now about the gap at the top and the gas. At original manufacture, an IGU will have a gas filling that is designed to keep it dry and assist in the insulating value. You don't specifically HAVE to do this, but if you have the ability and the materials, it will help. I already own a MIG welder and that has a gas regulator with it. The end of the gas line perfectly fit onto a coffee straw. I had purchased a welding bottle of 100% argon, and as argon is heavier than air - if you stand the glass up and have the gap at the top, the argon won't run out. Using a straw puts the "insertion point" deeper into the window and allows the heavy argon to push out the lighter nitrogen/oxygen air from the glass. Bottled welding gasses are also ZERO percent humidity, so even if you got a bottle of CO2/Argon mix, that would be fine. I let the regulator run the gas into the window for around 3 minutes, probably WAY longer than needed but it's hard to tell since you can't see it. Once I was satisfied, simply pull the straw out of the gap and squirt in a small amount of silicone to close the gap, and leave it to cure again.
Congratulations, your window is now ready to rebuild, and should never fog again. I've had mine in place now for over 2 years and it is still crystal clear.