That value of mainland is really kind of crazy. Old/Feted land still sells for amazingly high amounts. Check out the prices in the colored sims - they actually sell for that amount too. I know, I've sold off quite a bit of it lately. Now that my Ravenglass plot sold, I'm down to $0 / month... living off my free tier only.
I guess the + / - 40 meters on the mainland, coupled with the "nice neighborhood" really does add value.