As I said in my previous post, no other manufacturer does a exact oem self levelling shocker for the Omega - the Monroe version needs a reasonable amount of modification to the pipework and fittings to work, and I understand they have a tyre valve fitting as in the old style Cavaliers to manually alter the system with an airline. The valve on the Munroe shock is positioned lower down the body of the shocker as well which may need an extension to the original pipe work alone, never mind the connections and compressor side of things.
I did quite a bit of reasearch into the options when I needed to replace mine. I ended up with genuine GM and have not regretted it , and I tow a heavy trailer regularly, with genuine s/l springs as well.
Many of those in the link are just standard shock absorbers.
For the difference in cost, I'd try a few dealerships for genuine GM self levellers. You could be surprised at the differences in prices from dealer to dealer depending on whether they are in group stock or whether they have to order them from Luton or Germany.