There are many great lenses that will cover 12X20 when you get to the 19-24" and longer range. Virtually any of the apo type lenses cover nicely stopped down. This includes Apo Tessars, Apo Ronars, etc, as well ad Goerz RDA. You can often find these in barrel for not a lot of money.
With 12X20, and any ULF format, the premium is for wide angle coverage. If you want wide angle for any of these formats, expect to pay a bundle for it. For the 8X20 and 12X20, $3000 or more for the SSXL or $2000 plus for the 270-290-300mm Protar Series V, Computar, etc.
Dagors used to be great values for ULF, but now days they are, IMO, quite expensive for what that deliver.