Holy order tags Batman! Anyhoo, you can add a price to an order tag as seen below:
Instead of the price listing next to the order tag/modifier, you can have the price included in the item row by selecting “Add Tag Price To Order Price” under the tag general settings:
Did you know you can have the screen for order tags pop-up automatically for certain products when entered? It saved us a bunch of time when I found the option.
EDIT:
Have a look here, too, for an in-depth discussion: Using Order Tags as Product Modifiers