16 4mp varifocul cameras inside, 8 4mp varifocal cameras outside, 2 text inserters for cash registers, a 8tb NVR POE .
Be happy you didn't do the project for $4,200 from that guy - if he quoted you $4,200 for all the above he would have been using total JUNK.
I'll ball park this project for you @ cost level based on the above information.
NVR: 32 channel w/16 port POE ($500)
Hard Drives: 4TB WD Purple x2 ($300 total)
24 Port Switch with POE (For additional cameras and registers): $250
4MP Varifocals: Indoor Domes 16 x $229 Each ($3,664)
4mp Varifocals: Outdoor Bullets 8 x $299 ($2,392)
Security Cabinet for All hardware and wiring to terminate into: $100
Cable 2 Boxes solid core Cat 6: $125
Randomness (cable ties, connectors, additional mounts, blah blah blah) $200
So based on what you said above cost on this project for HIK hardware with 3 year warranty and next day replacement in US would be $7,531 (without POS Overlay hardware) plus shipping, so the $12k makes sense for this hardware spec out.
Now, I don't know your floor layout, parking lot layout etc but that is a LOT of cameras, also you can normally save money and skip Varifocal if you know what you are doing as you can order cameras with proper lenses for each camera location placement. While varifocal gives you the ability to dial in each camera for its coverage, you can do the same if you know what you are doing by specifically ordering a 4mm for one location and 6mm for another in the end spending the same amount for 2 cameras as you would for 1 varifocal.
I am always supportive of small businesses not doing work for free, and a lot of times people come here wanting professional for DIY pricing (NOT saying thats what you are doing). But it makes me even more mad when it sounds like you had an ASS HAT that doesn't understand what he's doing quote your project.
I'm in Memphis in a few weeks visiting a customer of mine, wish it was sooner as I would be glad to drop in and make recomendations.
What POS solution are you using?