I initially said the 3 tiers just to keep things simpler...but since I'll be tracking everything anyway - how about this compromise, subject to j3sus's approval of course:
12x12 initial plot is free (or 20c if j3sus wants)
every 1x1 extra area will be 30c each, up to a maximum of 24x24. This way, the max 24x24 space would only be an extra 360c. We could discuss plots larger than 24x24 on a case by case basis - but just to provide a limit we can stick with that as the max for now. That way, if existing houses are just a few over 12x12, or if a particular house design is 15x15, people can buy exactly what they need. And just to clarify for people, the 12x12 (or extra land purchased) is for the plot, not just the structure. So if you build a house and a fence around it marking off your own territory, the edges of the fence would be the edge of your plot. Not just at ground level, but also above ground and below ground.
Also, if I sound strict about things in general, it's not because I wanna be a jerk or penalize people, or think that I'm better than others, I just think strong boundaries are good for keeping people in check. And they're not necessary for people like you Plasma, but they are necessary for younger, more casual players, that don't really care about the town as much.