To add prices to the a modifier please go to the backend of each store that you wish to add them to, click "Settings" → "Modifiers" then scroll down to the modifier you want, for this example I will show "Kids Sides", don't go to the group of them, but the individual items. I attached a photo for reference.

After you find them, you will see that some sides do not have a price next to them, these are the ones you will need to change. Click on the one you want to add the price to and it will pull up the page for it. See photo below.
Inside the "Price Type" put "add", then inside "Price" put the price you want it to be which for this example is "1.50". It should then look like this.

Make sure to scroll down and press save.
Repeat for all sides and stores and you will be all set!

Let me know if you have any questions on this