Yes, but art also has a set value based on numerous factors. The most important being the name of the artist. That's why pieces of art get appraised, to determine their value.
You mentioned Picasso. Compare what his work fetches at auction to a completely unknown artist. Is it the same? No.
You can't compare stuff in this hobby to traditional art. It never works. No one buys stuff in this hobby because of the person who made it. They buy it based off the finished product. Who actually made the piece is irrelevant. It's the opposite when it comes to traditional artwork. People will spend millions to own a piece of artwork from a particular artist just to say they own a piece of artwork from a particular artist. They don't have to like it.
It's Apples and Oranges. And it'll be that way until a "forum customizer" can crumple up a napkin, paint two crappy eyes and a smiley mouth with a sharpie and sell it for hundreds of dollars simply because people want something, ANYTHING from that artist.
Now beyond that, I think the reason people are kinda put off by the price is because this isn't an original, 100% built from scratch figure. This customizer is essentially selling other peoples hard work (as well as his own) and trying to make a profit. He didn't sculpt that head, or make the dreads, or sculpt/design that body, or sculpt the majority of that armor or make that base. Hot Toys did all that work.
Concerning your Acura tl type s: Think of the price you paid for it. Now imagine someone repainting it, adding a few minor things and then selling it for a minimum of 4x that original cost and up to 6x that original cost. Would you still agree that's a fair price for what you're getting?