I think the price came down quite a bit from six months ago or so. Back then I saw a pair go $3000+ without the case. Later model numbers definitely command better price, one post-2004 set was sold $4000+ few months ago.
There is no "fair" price. Since practically all R10s on the market will be used that attribute will not be a large factor for most people. Everyone has their own price and the supply of R10s on the used market coupled with the condition of said R10 will determine what the individual will pay. Some folks dislike the R10 and would rather buy a (much cheaper) K1000 while others would easily pay over $4500 (some prefer the R10 to the $5000+ HE90 and I could certainly empathize with those people though I personally prefer the O). If I was a huge fan of the R10 and the 'phone was in excellent condition (most R10s have been gently used by their owners but I've seen a couple heavily used models) I would pay over $4000 if I was searching for it for a long time - the serial number (some prefer the later releases) is another issue that could affect the resale price. The "fair" price is what someone is willing to pay for the R10.