How much does it cost to repair a 12.9-inch iPad screen?

That will depend on the generation of iPad Pro that it is, whether you have Apple Care+ or not and where you are in the world.

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

click here ➜ iPad Repair and Service - Apple Support

For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair while expensive will be somewhat less than the cost of a replacement iPad.

As for out-of-warranty repair costs, Apple offers a fixed-price price repair. You can verify eligibility and cost for your region/country here:

iPad Repair & Service - Apple Support

If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.