"At piggy-bank prices" refers to something that is very affordable. It typically implies that the cost of the item is low enough that it can be purchased with the money saved in a child's piggy bank. There are several synonyms for this phrase, including "inexpensively," "affordably," "cheaply," "reasonably," and "economically." Other ways to describe a product that is sold at piggy-bank prices could include "budget-friendly," "low-priced," or "cost-effective." Whatever the wording, the idea is to convey the notion that the item is a great value for the money and that it won't break the bank.