Hi,
Price is change, if customer order service add-ons over limits.
For example, you have service add-on like this
http://monosnap.com/image/BlA0hhyoq9...d3kO73XN9iFPpI
If customer during order process set more the 1000 then price will be changed according add-on price.
But if customer do not change default value 1000 the price will not be changed.