Name that fallacy: If it's more expensive it must be better

Solution 1:

The name of the fallacy is appeal to wealth or argumentum ad crumenam.

An appeal to wealth (argumentum ad crumenam) occurs when more money involved means something is truer or better, exploiting the impression that money flows from intelligence or work.

_{Source: http://rationalwiki.org/wiki/Appeal_to_money}

Solution 2:

You say "fallacy", I say "heuristic". The "common law of business balance", sometimes expressed using the aphorism "you get what you pay for", expresses a correlation between the price of a good and its quality. When a buyer has no other evidence of a product's quality, the buyer is likely to assume that the seller of the less expensive product cut corners that negatively affect its quality in some material way. So the user is applying a quality heuristic.