At first, I was looking on eBay at Rolleicord Vs, the prices, though, for the good ones are unreal. A 'mint' condition Vb recently went for almost $500USD. Even well worn ones in need of a serious CLA are selling for about $150USD.
I have found that Rolleiflex Automats such as the MX model sell for significantly less ($150 - 200 USD)than the Cord V, Va, and Vb. Optically there should be little to no difference, as all of these cameras use similar optics (Schneider Xenar or Zeiss Tessar). The differences between an MX and a Cord really boil down to ergonomics in use.

If you are using a Cord II with Triotar lens, you will be pleasantly surprised at the level of detail from either the Xenar or Tessar lenses, especially when stopped down to f/8 - f/11. Although, to be honest, I find it hard to tell the difference between the Tessar on my MX and the Xenotar on my 2.8D at f/5.6.

If economy is a factor (and when isn't it?) the smaller f/3.5 camera is the way to go, as Bayonet I accessories (filters, caps, lens hoods) are more plentiful and much cheaper than their Bay III counterparts.

Make sure to budget for a lens hood and strap. Flare does tend to be an issue with these older single-coated lenses, and the MX model did not contain light baffles behind the taking lens until the last 50,000 manufactured. The left-handed focusing on the Flex models makes the strap a requirement unless you are an accomplished juggler.

If you want to go really cheap, consider a Rolleicord III with Xenar lens. These should be available for around $100 USD and use the Bay I accessories. I have one of these and frankly, I find it the most ergonomically logical of my three Rollei TLRs. My sharpest handheld shots were made with this camera even though in theory this should be the softest of the three lenses.

If interested in seeing some low-res web images from my Rolleis, point your browser at:
http://homepage.mac.com/oboeaaron/rolleipics

There is a very active Rollei TLR/SLR mailing list to which I belong. If anyone here is interested in subscribing, send me a PM and I'll send instructions along.