The dollar sign is redundant and a constant reminder that the customer is giving up dollars they already have for food of unknown value. Humans are also hardwired to value a thing they already have more than something of equal nominal value that they could easily acquire.
In the same way that freemium games elide your spends of real money behind a layer of invented virtual currency, the goal is to make the customer forget that they are spending actual money.
In the same way that freemium games elide your spends of real money behind a layer of invented virtual currency, the goal is to make the customer forget that they are spending actual money.