In my opinion, 20 bucks is an absurdly low price for what is being offered. And until ALL the land is sold, you can feel confident that the cost per plot will never rise above that. Buying land over the next 12 months (in any quantity or frequency) is a great deal--for as long as $20 land is available to buy.

Once all 150,000 plots have been sold, I predict the cost for a single plot (on the player-driven marketplace) will skyrocket. After the land expansion has been implemented in the game, people will quickly realize what they have and treat each one like a golden goose.