I recently picked up a Bronica ETRS body, prism, back and speedgrip for $125 off Ebay. Now I'm in the process of shopping for a lens. The most common lengths -- 75, 150, 50 -- go for a song. The less common for only a little bit more. I painstakingly compared prices between KEH (which I love and trust -- always buy BGN grade stuff from them with no complaints) and Ebay. In most cases KEH is the same or cheaper than Ebay and includes warranty; however, occasionally you can find something that turns up on Ebay for such a cheap price that it's worth taking a risk as it was in my case.
Since it's a modular system, there's no harm in buying one or two components off Ebay if you happen upon an extraordinarily good deal, and then filling out the rest of the system on KEH. I personally wouldn't bother with the older Bronica S or EC cameras to save money unless you just especially want one -- they are around the same price as the ETR series. The Bronica SQ cameras are only slightly more than the ETR series, but not quite as much to choose from in the used market. Bottom line: $300 will easily get you a kit...less if you are a little patient on Ebay.