Final answer:
The principle that indicates the lower-priced house will generally be purchased first is supply and demand, according to the law of demand.
Step-by-step explanation:
If two similar homes are for sale, the concept that the lower-priced house will usually be purchased first relates directly to the principle of supply and demand. This principle is rooted in the law of demand which states that a higher price leads to a lower quantity demanded and conversely, a lower price tends to result in a higher quantity demanded. Applied to the housing market, when all other variables are held constant, consumers will gravitate towards the lower-priced option, especially when it compares favourably with other, more expensive, similar options.