How much would it be to fix iPhone 12 front and back

Without Apple Care Plus it will be 2/3 the cost of a new phone and I know which I would rather have,

Screen Repair (front or back)

iPhone Screen Repair & Replacement - Apple Support

You could likely buy a refurbished iPhone 12 for about the same cost as repairs.....and the refurb phone will come with a new 1 year warranty.