85's retail for around $3,000. Some a little over, some a little under. The KX-100 new retails for $3,250.
But you can look for some leftover models and save tons of money. I bought my brand new leftover 2003 KX-125 for only 3,800$ in total. My friend bought a brand new 2003 YZ-125 for almost 6,000$ in total! Both bike retial at $5,000 but with tax, shipping, prep, all the BS they get you on, your looking at a lot more money. My bike was 3200, and came out to 3800. What a deal.
The RM-100's sold like crap from what I hear (and from that there is no 04 model, only kx-100s) My friend was looking at this 03 RM-100 around here for only $2,700. But he got a better deal on a combo of a KX-100 and KX-65 so they got that. But anyway, look around, there are all sorts of great deals out there.
Even used, such as a KDX 200 would be nice and you can get a nice one in that price range.