I don't have anything that uses this tube, but there is an RCA on ePay for $.99. I've liked them in other things.
Anything above $5 is too much for a 6GK5, but that is the problem Sir Thrift-a-lot. If one cares to roll the typical 12ax7 or 6dj8, all the top examples are codified and up for sale on epay and you just missed out on this great deal:
Vintage/NOS Genalex Gold Lion B759/12AX7/ECC83 Matched Pair!
$2,402.17
14 bids
+$5.95 shipping
(I happen to have one of these used and can roll it in the power supply of my current preamp, where I've rolled most of the top 12ax7. As you can see, this tube rolling can get out of hand.)
With a 6GK5, sellers feel put out just making the effort to list one. Typical tube dealers will just have an entry 6GK5, $5. They won't even break them out each brand into seperate entries.
So 6GK5 tube rolling is not known well and i bring my images of the humble slant getter Japanese 6GK5 as a scoop of sorts for the local denizens.