Unless Mike changed it this morning, it shows on my computer as $450 for the M6, $525 for pinstripes, and $650 for nickel core. Any chance you are not looking at the
http://www.magnequest.us/ site? I think there are one or two obsolete sites that are still online - at least there were a few years ago.
Yes, the BH-5 is a variant of the TFA-2004Jr. Mike had some proprietary ideas to make it more affordable and improve the sonics at the same time; these particular designs are only available in the Bottlehead upgrade kits. The differences are most audible in the high treble.
Note that the full-size TFA-2004 in nickel is $700/pair without plate choke. The Jr is not currently listed but a batch is is occasionally produced. IIRC the matching BPC choke - no longer listed - went for $450/pair when it was last produced.