jumping in - few aero ektars on ebay right now and recent ones went for £150-£250 ish,
however if you dont want to make your own lensboard for it, you need one like the JoLo one included in that sale, which were about £80 but are no longer made and have all been sold... there is another chap who makes them, check out the dancing with speeds facebook group...
the jolo board also works with the pentac 8" too which is useful.
so although £400 seems a bit over priced maybe not as much as I originally thought...
surplus ones were sold off for $10 apparently back in the day..! cheers