Did I just get ripped off? How do you know if it's a good price?

I live in Japan, I'm from the US. My husband and I agreed we would like to take things back to the states, such as a pair of shisa or artwork created by a native. When an elderly man came to our house selling paintings (he'd been going from door to door), we thought they were beautiful and bought one priced at $250. It looked like an authentic painting, but then what do I know. I am just hoping we weren't ripped off or that it wasn't overpriced. I've seen some similar paintings (I want to say identical), maybe he sells to that store? How much should a medium sized (no bigger than 2 ft by 2 ft) oil painting cost? I hate to ask, because having come to know the Japanese, they give me no reason to suspect them of cheating me, but I'm curious.