Who is the seller? Palestinian?
Secondly, are those houses cheaper than living inside Israel? That's why they tend to sell?
What do the people who buy these houses do as a profession? Commute inside Israel to work or do they farm in the occupied territory?
The land is a mixture of land sold by Palestinians and land that is seized by the Israeli government for "security reasons".
The houses are cheaper because of location (far from the high-demand cities), risk (of attack by Palestinians and of land boundaries being adjusted in a peace settlement).
However, price doesn't drive their sales. The settlers who live in the settlements are motivated to live there by religious ideology.
Some settlers farm, others commute. The Israeli government also funds and encourages economic development in major settlements to ensure local jobs for settlers.