It depends on what specifically you're ordering, some of it is gonna get a little fuzzy, bear with me, we're gonna get in the weeds a little.
Tldr generally it that is doesn't work like that
Essentially, it breaks down to costs, what's a mixed drinks, what's a cocktail, who's your daddy etc etc
You referenced a mojito so we'll start there. The bar has already priced out the mojito so have some quantity rum, some lime juice, some sugar, ice, soda, mint leaves, and probably the time it takes for the bartender to make the drink.
When the bar does the pricing, they're assuming the bar tender is gonna use well rum. When you call for zacapa 23 (aside: I hope you don't it's not gonna taste very good) they're gonna fudge the price up based on how much more expensive the rum you've called for is.
Generally, it's not additive like 18 +12 because you're substituting not adding, but it is gonna be more, everyone's costs are different, so shooting from the hip, guessing, it's gonna be 18-22$.
"Mixed drinks" are different from "cocktails" on how low effort aka fast aka cheap they are to make. So if you order a zacapa soda or a zacapa and 7, generally they're gonna charge you for the shot price because it's so easy to make and the inputs are so cheap.
A good rule of thumb is if you're asking for more than 2 ingredients or anything more complicated than thing goes in cup, it's a cocktail