I found a used tiller - JD 450 Rotary Tiller - for 1K.
http://www.deere.com/en_US/govsales/purchaseguide/PDFs/100_101.pdf
It weighs just over 300 lb. and it has a slip clutch so I am thinking even though it is a little bigger than I need (50" wide), it should be ok for my tractor - BX1850...