Well, it depends. I bought my RZ67 + 120 back + WLF + 90/3.5 for 350 euros last summer. I thought it was pretty decent price. Recently I bought a 250/4.5 for 115 euros, so you just need to keep looking if you find the prices high.
Huge fan of the 50mm. I'm a wide angle slut. I also have the 250mm, and love it. The only other lens that I jones for is the 37mm (I think that's right) fish eye. Like I said I'm a WA slut.
My most used lens on my RB67 is my 50mm C, and my second most used lens is my 250mm C. I also have a 90mm non-C, a 127mm C, and a 180mm C for portraits. I borrowed my friend's 65mm C, but I didn't like the AOV as much as I do with a 35mm lens on 135 format (the 35mm f/2 in 135 format is one of my favorite lenses).
I too, started with the 180mm first and purchased the 90mm second (bought at retail prices) and used them for portraits mostly. A few years later when used lens prices were a bargain I bought a few more in this order: 65mm, 250mm, 360mm (w/2x converter), 140 macro, 100-200 zoom. The 50mm never attracted me because I don't like image distortion (when I go for a wider view I prefer a 6x8 view camera w/65mm and 47mm lenses - for more control).
My favorites, and most used, are the 65mm, 180mm and 360mm. The 180 is the sharpest of all (KL lenses). The 250mm and zoom are hardly ever used because they add too much weight to my backpack.
If I was to choose only two lenses it would be the 65mm and 180mm (lightest of all) and maybe an extension tube. Very useful focal lengths for general shooting.
I am actually looking for a 4th lens for my system. Currently I have 50mm, 90mm and 180mm. I am looking at the 65mm but I'm also looking into the 250mm. At the moment I mainly use my RB for landscape work, however, I will soon be doing more studio work. Which one do you find more useful overall?
For studio I'd think the 140 macro or the 150 SF would fill an important void in your lineup. The 250 does sound very nice too. Ah hell, like I said before, you'll want 'em all eventually!
My RB came with the 127. Then I added a used C180 and a new KL90. Today I found a very cheap used C250. At some point if I find a great deal I'll probably pick up the 65 too. I have only had the camera for about 3-4 months and have only shot 4-5 rolls of film so I still don't know which lenses I like best.