I thought that most of the cost was in the ballasts themselves. Perhaps you'd save money if you were building a smaller (10x12/8x11, etc) printer as opposed to a giant bank of spiral bl bulbs.. I'm thinking of building one, myself and using the spiral bulbs.
I only really print 4x5's at the moment so I wouldn't need a 16x20 or 20x24 monster printer.

has 27w BLB for $12.50.

Item #: SL20/BLB-BX, Catalog #: 05439
is a 20w BLB for $11.49.

Does anyone have any experience or tips? I am interested in this as well (and sorry if i've hijacked a thread)