How much will a top screen replacement cost for the 8th gen iPad?

See the "How Much will it cost" section in the following support document.

click here ➜ iPad Repair & Service - Apple Support

Apple only provides the cost of $29 for screen replacement on the iPad if you have AppleCare coverage. In most cases, it is not cost effective to replace the screen without AppleCare, so it is best to take it to the Apple Store to see what your options are.