What do you want out of your camera? I think that there are two major issues with Rolleiflexes. One is condition. Pure and simple. Old cameras need to be cared for.
Second, lens. There are two main classes- the 4-element Tessar/Zenar lens, and the 5/6 element Planar/Xenotar. The Tessar-type is on models like the MX, MX-EVS, and Rolleicords. Almost all 'lettered' Rolleiflexes (C,D, E, and F) are the Planar-type. Your YashicaMat has a Tessar-type lens. The Planar-types tend towards better microcontrast and sharper.
By the time the MX-EVS was being made, Rollei had pretty well established the basic mechnisms through out the camera. There was some finessing of things like the focus adjustment and alignment system and such, and metering and couple metering, but no real structural changes.
Generally a Tessar-type will run you $250-550, a Planar-type will run you $500- 1200 and beyond
I do have an MX-EVS in the classifieds still.