Chameleon's answer is spot on, but their explanation isn't entirely accurate. The statement
| an adjective ... cannot be the object of a preposition |
|
isn't always true. For example, "this is
beyond ridiculous" is a counterexample.
Nonetheless, "for cheap", doesn't make sense, because the specific preposition "for" does not take an adjective.
You can, however, say "for less", because then the sentence would
really be saying "for less THAN SOMETHING". In fact, this would be a good marketing gimmick, because you're not actually specifying WHAT it's less than. You might mean "for less than a trip to the moon" or something, but your customers are
bound to infer "for less than one expects". (Marketing is one profession in which accurate communication is a positive hinderence, and the real goal is to place a lie in the listener's mind, without actually lying). But you're buying, not selling, so I wouldn't recommend this.
Rommie