Well, start with "low prices". How low? Very low - down to the ground! BUT you can go lower than that! So dig down... now the prices are "lower than the ground". Very low indeed. Subterranean. And then... you hit rock bottom. It is not possible to go any lower. Beneath is solid rock. So "rock bottom prices" is "the lowest, lowest imaginable price".

There is an excellent website that defines almost 4,000 idioms and expressions: http://www.usingenglish.com/reference/idioms/ I searched for the expression "rock bottom prices" and there were no results. Then I searched for "rock bottom" and got the following result: "Hit rock bottom" When someone hits rock bottom, they reach a point in life where things could not get any worse. The expression "rock bottom price" means the price could not be any lower. If you do not find the idiom or expression on the website, just Google the expression and you will find many results.