I collect backcards like an insane creature. I think last count I had around 250 different ones. Some are more hard to find than others. Generally non-US cards are rarer so I find them more expensive. $1-$10 is probably a good benchmark for most US cards, I think. Depending on condition and the pony. Mimic's might go for more
I dunno. It probably does relate to the pony.
I sometimes pay more than market price for a particular card if it's one I'm looking for. But I have heard people still talk about recycling them or tossing them out even now. So I think it's a specific thing for a collector xD. For me it's my nirvana, but I think it's a bit of a niche obsession...
My criteria for backcards...
If it's common or semicommon, I'd rather it wasn't ripped or missing parts (although horseshoe points sometimes I don't mind). The condition and the price here are interrelated. If it's rough condition but low price sometimes I will go for it
If it's medium rare then I'm a bit less picky about card bending and damage. It also depends on price here too.
If it's super HTF, like some of the UK ones - eg Mountain Boys, 1988 twins, etc - then I'll take it in whatever condition it appears in, because the chances are I won't get another any time soon.
For me it's a mix of art and history, especially with the UK ones, because they are often pieces of the information puzzle of what happened in the UK line.