Cab-1 and command base TMCC sets do show up for less than $300 - I found a new set at a local meet a couple of months ago for about $175, and I've seen them on Trainz for well under $300. The auction sites are crazy, though.
Major caveat: If you see one for a bargain price, make sure that it includes the power supply. The power supply might look like an ordinary wall wart, but it isn't - it possesses magical powers. If it's missing, there are options - search for threads regarding TMCC power supply for further details - it's been done to death here on the forum.
