How much will it cost to fix a cracked/led screen.

It's not clear that you are getting the purpose of this entire website. We can't tell you how much it's going to cost you to fix your phone, since the only cost to you is what you pay to buy your own parts in order to fix your own phone. iFixit provides repair guides for thousands of devices, including your phone. For example, here's the guide you would need to replace your screen.

iPhone 12 Screen Replacement - iFixit Repair Guide

iFixit does also sell parts; in fact they have three different options for your phone; a cheap LCD conversion, a mid-range hard OLED and a higher end soft OLED. So the cost is entirely up to you as to what you want to buy, and from whom; obviously iFixit isn't the only site selling replacement screens.

iPhone 12/12 Pro Screen: LCD + Digitizer Replacement Part, Kit

Should you decide to buy your screen here, I'd recommend the Fix Kit version, as it also has all the tools and adhesives needed to do the job right, and it's only an extra $5 or $7 USD, depending on which screen you purchase.

You should be aware that Apple started pairing the display with the logic board starting with the 3rd gen iPhone SE, the phone released just prior to your 12. As a result, replacing the screen will result in a message stating that Apple cannot determine if your screen is genuine - even if it is. You will also lose the TrueTone function. The only way around these issues is to either use the Apple self repair program, or buy a genuine Apple screen and use the Repair Assistant available starting with iOS 18 to pair the new screen, which will also reenable TrueTone.