Looking at recent pricing trends, I think most 10th are around $100 cheaper than 8th versions, though there seems to be some heavy discounting in 10th scale recently from international sellers.
As for parts, I vaguely remember the on line retailers rolling out parts as little as a week or so after the first models came out?.
I'm actually very keen to see if Mugen can pull back a lot of it's market, so many have become either Xray or now Capricorn pilots, me I made a decision to just hold with the 4 till the 5 came out, but then I'm not a major racer either.
(I did almost buy a Serpent though, just cause I hate working on the 4)