After some early dabbling we've always shied away from building an economy into Mini Metro, it's not where the strength of the game is. However this is the first suggestion that I could see fitting in nicely and not being too burdensome! At the moment we try to have every upgrade be worth an equal amount but this would give us a lot more leeway in designing new ones. I don't think we could make a change this major to Mini Metro, as we'd have to overhaul the entire upgrade system to make it worthwhile, but it is something to think about for the future.
I think that increasing costs would encouraging a convergence of play styles though. It's nice to let people freely explore what a game is like with only carriages, or only extra lines, or no extra tunnels, and having upgrades increase in value would make that more difficult.