A mining card can run through a PCI -> PCIe adapter, even via Extender cable, without issue.
Yup i can confirm this
Hell it even works with PCI-X (yes X not E) because it's backwardscompatible with PCI...
As for the price: I found some on ebay for 25$ which isn't too bad of a price if you already have some old hardware lying around like me, for example i converted a old fujitsu siemens rack server to a mining rig with one of these adaptors, an extender cable and a 5850
If you're having problems getting the board to detect the card you'll probably have to do this:
just on the adaptor itself, at least that worked for me