We're starting to get away from your original question. The Prelude is an odd duck. It lacks the features of it's predecessor the E-50 (at a much lower price point.) It's almost like it was designed by a different team of programmers, out of step with Roland's usual product release cycle. I would guess that current draw is low enough to run on batteries. It has a decent sound set and Roland's excellent, highly logical chord recognition. It also has a fairly small form factor as arrangers go (but not feather light like the newest Casio's.) I don't believe that many were sold so it's hard to say what a fair price would be.

The Prelude has an on-board 16-track sequencer whereas the BK-5 does not. Except for that, the BK-5 is probably a little better in every other way. Maybe Diki or someone that owns one can add more.