Land price is calculated approximately by 600*[1.1^(total land bought)]. This approximation gets increasingly inaccurate, but with about a 50% error for 100 land when a gap of 5 fills it isn't half-bad.
Prices of every land are higher than the previous one by about 9.61757%.
We also know that if the amount of land owned < 10, then the land price = 0.