Which of the following is the least expensive liquor modifier?

The least expensive liquor modifier is 'Up.' This term is used to describe a drink that is served straight from the shaker or mixing glass, without any ice. When ordering a drink 'up,' the presentation is often more elegant, and because it has no added mixers or ice, it does not inflate the drink's cost with additional ingredients.

In contrast, the other options involve different serving methods that may add to the complexity or volume of the drink. For example, ordering a drink 'Neat' means it is served at room temperature in a glass without ice, which is also a straightforward way to enjoy spirits, but it may still carry slightly different pricing based on bar policy. The 'Rocks' modifier indicates that the drink will be served over ice, which can dilute the spirit somewhat and often results in a larger pour, potentially making it more expensive.

Offering drinks 'Up' typically avoids the cost associated with ice and additional mixers, making it the most economical choice among these modifiers.