Question How much should i pay for an used Twin frozr gtx 780 3gb

Well, if you take a look at the GPU hierarchy chart, and scroll down to the Legacy Chart section, you'll see that the 780 is roughly on par with:

  • Nvidia:
    • GTX 780Ti
    • GTX 970
    • GTX 1060
  • AMD:
    • HD 7990
    • R9 290
    • R9 290X
    • R9 390
    • R9 390X
    • RX 470
    • RX 480
    • RX 570
    • RX 580.
Comparing to cards still available new, with warranty, currently in the US you can pick up a Sapphire Pulse 4GB RX 570 for $119.99.

The 780 is a used card, with less memory, and a pretty obnoxious 250W TDP. I don't think I'd personally look at it unless it was half the price of the RX 570, or less. Maybe that makes me a cheap SOB.
I used Germany as a guess, and the cheapest was an 8GB RX 570 for €147.84

Most other countries in Europe that are selectable on PCPartPicker are pretty close to that, though specific models and whether it's 4 or 8GB differs.