Yeah, if there is something that will break the plugin interface, we give the offending release to the plugin writers first and give them a chance to prepare a bit.
But the API changes that break the interface are few and far between now. I hope that soon we just won't have that anymore. But nothing is forever, so I will never be one to say that it just won't happen in the future.
Cost of a plugin... well... it is rather difficult to pinpoint that. What with the old 80/20 rule. 80% can probably be done real fast but the last 20% is the killer. Paying a programmer to do it will probably cost in the order of 5 to 10K. But probably a lot more by the time it gets production ready. So ROI on it at $25.00 a license (guessing?) is going to take a while. Even if the plugin takes off like wild fire, it would take a bit to recoup the investment. The only way out of that is to raise the price. But it would really be a hard sell to have a plugin reach the cost of Mach to begin with!
Take Darwin, for instance. Art will never see any break even line on that. He did it because he is Art and that was enough for him. He did it for the love!

Steve