To say "it fits on the back of my B6100E" indicates that it is a 3pt hitch type finish mower. I know that Kubota recently came out with a new line of 3pt finish mowers; I don't know anything about their history in that area.
Big D, for anyone to help you, you'll have to be more specific. Is it in fact a 3pt hitch or mid-mower (a mid-mower is very limited as to the tractors that it would work with)? What is the cutting width (48", 60" or other)? What company built the mower? Is it the newer style finish mower with 4 wheels to adjust the cutting height or the older style where the 3pt hitch position detemines the cutting height?
From my shopping perspective, brand new 3pt finish mowers run from $800-$1600 (sometimes more) depending on a lot of different factors (mostly name brand).