The BG 55, 65 and 85 all share the same engine. No, there definitely not more than $5 differnce in manf cost bewteen the 3, but that's pretty common with all "same family" devics. Does it REALLY cost that much more to make an 440 than 260 (a little)? or a BR340 to a $420 (zero), or an 290, 310 or 390 (close to zero)? na... it's what the market will bear. Stihl or the dealer isn't making squat on the BG55 at $139 MSRP..
As for the differences... wish it was as simple as just changing the muffler screen insert (oh - it unscrews, so real easy to "port" the muffler), but the fan casing (entire support body) is different on the BG85. Once they (55,65,85) spin at 6800-7200, you aren't going to get any more air out of them without design changes.
Me? I hate hand helds of any brand (except on my roof), but I do have a big property. I'm getting the "to be announced" turbine blower attachment for the KM units (early next year?). From the pics it looks about 16 inches long, 4 inches in diameter, and mounts right on the KM connector. On a KM55 it has the performance of the BG85, and on a KM130 it has the same performance as a BR550... or so they say.. ($129 msrp..)