Low cost + interchangeable lenses screams "Mamiya TLR" to me. See Mamiya C33, C220, C330 variants. I have some examples in my gallery of C330 with 80mm lens. I have some other lenses but to be honest I almost never take the 80mm off.

Though I must say, I occasionally see a Mamiya RB67 SLR with lens for under $200 on Craigslist. I don't own one (yet) but I imagine them to be a bit heavier than the C330. I could be wrong on that. As TLR's go I think the C330 is on the heavy side because it's a lot more complex than a Rollei or Yashica with non-removable lens.