I've seen many pairs go for $250 and up if they are nice and the auction is well-written with good pix, etc... That is cheaper than 100s, but not much.
HPM-100 sounds a touch "tubby" to me,... but in a good way. Big warm low-end. The HPM-60 sounds a little more even top to bottom and replace some of that phat bottom with more nimble and punchy bass. Personally, I prefer the 60's (at ANY listening level) but I'm probably in the minority.
I'd say that they sound 95% alike. If you like one, you'll like the other.