The cheapest entry into medium format would be a folder, like the Agfa Isolettes. However, you will be without metering and coupled rangefinder, so it is truly manual. Skipping the Holgas, the next step up the ladder are the TLRs, as suggested by several posters. After that you will find yourself in MF SLR land, with a wide variety of choice. The cheapest in this category are probably the Bronica ETRS series. At they start at around 300 USD for a kit with body, back, lens and either waist level viewfinder or prism. After I bought mine, I wrote a small post on the considerations I went through. You can see it here on apug