I’ve been considering making custom backgrounds, but I have no idea what I’d price them for. I understand that quality matters, but other than that, what’s reasonable? 5-10 gold? More? Less?

I would recommend a range of prices and amounts of work put into each.

You won't get many players who can afford gold, so weight your range toward silver. Make half your backgrounds a high silver price, and most of the rest lower. Then one or two gold, and one or two, say, 3000 silver for the newbies. Then see what sells.