What will be the cost of an iPhone 14 Pro back panel?

Prices are not published because everything changes by region and country , get your repair estimates from this link

iPhone Repair & Service - Apple Support

According to the estimating tool on the page linked to above, this is what you would pay in U.S. dollars:

When you open the link from your computer, it should take you to your region. If it doesn't, you can change the region at the very bottom right corner of evey page.