Well, it depends what to consider as good.  Most
of the ones that are floating in the market are
profile ones of Tomoe.  Prices range from $150 to
$250.  I have seen a nice front face view of her,
it sold at around $400.  

It's pretty hard to find RK OVA cels because most
of the scenes are done in computers.  Certain
scenes are done by some other smaller studios, 
that's why they're done in cels.