The first sword I ever paid money for was a q9 insc Fellblade (I think I paid 50k), and that was when I had about 60k in total to my name, and it took a long time to save up back then.
Currently yes, 15k is about the going rate. However I should mention that often it is still possible to sell for 30k, there are many buyers who pop up in Kamadan seeking a Fell. Remember too that 10-15k is a "clean" Fell, expect to ask a little more for one with perfect and desired mods.
I have maybe a dozen Fellblades on heroes and such, love them. In fact my q9 Obi Edge is on Koss, I use a Fell on my Warr
* EDIT: If you are not in a rush, I would ask 25k for yours. It may take time to sell (and don't spam), wait until you see someone asking for one in trade. If not bang it out the door for 15k as others have suggested (unmodded) and sell mods 4k, 1k, 4k.