Comment below rating threshold, click here to show it.
When I went to buy a Greater Mark of Magic Penetration, it initially displayed a cost of 0 for one rune. Incrementing the quantity counter to two it displayed the proper price of 820, then decrementing it again to 1 would then display the price as 410.
I purchased one rune and it did charge me the correct 410 IP. After purchasing that one rune, I went to buy another, and I could no longer operate the + or - buttons for the quantity counter as they were grayed out and not responding to clicks. This only happened with Greater Mark of Magic Penetration, none of the other runes I tried.
Attached is a screenshot of this happening and a DxDiag report. I hope this helps and please ask if you need any more info!
EDIT: The first png screenshot I uploaded got scaled down a lot! I re-uploaded it as a gif. Sorry for the bad compression on the image.