How much to replace the windscreen should be the first question as that defines how much is for the rust repair.
As already mentioned by vc_commodore, that could run out to $400. So the question then becomes what are they going to do for the $1600 you‘re paying?
Will they unpick the spot welds so the whole piece can be removed, clean up and then repair it before welding it back to the vehicle, or just bog it up or something in between? If it’s the first, then that price is rather good, it it’s the later, I’d do it myself.
Me, I’d try and take out the windscreen and and if it breaks it breaks. Then I’d go about cleaning up the rust area with a wire wheel. If you haven’t got any holes, you’ve hit the jackpot so just paint it up but I’d suspect pin holes at a minimum. If you’ve got pin holes you could just braze them up (remove combustible, meltable stuff from the area you are working on before hitting it with flame). If you’ve got massive holes, you should cut out or unpick the offending part and repair it off car with new steel and clean up the welds on both sides.
The problem is you need to also check where and how water has been getting caught in the area and the condition of the pillar internals (the bits you can’t easily see) and whether they are also rusting. You may need to resolve these water entry issues and any hidden rust (which is why it’s best to unpick the panel to gain good access. This IS something that can be done at home if you have some basic welding skills and aren’t afraid of jumping in, but it is time consuming and you‘d really want to keep the car to go to such effort.
Much easier to ask the panel guy what he will be doing and to check up during the various stages of the repair process. That way your satisfied you’re getting your money’s worth and he’s actually doing what he said he’d do. But again, you’ve gotta wanna keep the car and be happy with the money you’re spending on it.
Ps: google A pillar rust repair and you’ll find lots of info and YouTube videos on what to do and what not to do. here’s one article of rust repair of old cars.
https://www.hotrod.com/articles/repairing-rusted-a-pillars-the-right-way/