Perversely, the 35mm Soft Focus lenses are usually more expensive than the Medium or Large Format lenses. That's probably just supply and demand, with most supply coming from wedding photographers ditching their RB67s and P67s and the demand coming from people with smaller cameras wanting to try something different. Some of the really rare LF lenses are getting high prices (Pinkham & Smith, Nicola Perscheid) for the optics that will cover 8x10 and larger, but most of them are in the $500 - $1000 range and the most recently produced Fuji SF lenses (an Imagon style lens) can usually be found in the latest style shutters for under $400.
The Pentax 67 120/4 soft is getting a little harder to find, but will probably cost less than $400 and there are quite a few adapters out there now to mount it on other systems. By comparison the Pentax 35mm soft lenses (FA28/2.8, 85/2.2, F85/2.8 and FA85/2.8) are rarely found for less than $400 and usually have price tags in the $600 - $800 range.
One thing to understand though is generally a specific soft focus lens is capable of quite different things to just adding a filter in front of your normal lens. There was a lot of experimentation and some quite different approaches taken. In the Pentax 35mm line the 85/2.2 is a completely different construction to the 85/2.8. There is no right way to achieve it since the desired result is an aesthetic effect and what is beautiful to one person may be undesirable to another. Which is probably why people who get the SF bug end up with a lot of different lenses.