Well, let's start at $200/teacher. That would $660,000,000.
However, let's do a little research and logical thinking...
A quick google search produces this:
Active Shooter Training
This class costs $500/participant and is a 2 day class. It assumes you have some prior training and own a weapon.
Well, many of these folks don't have training so...
Fighting Pistol is a 2 class that covers using a pistol in a combat situation. Again, you need to own a weapon, but also bring 1,000 rounds of ammo and eye/ear protection.
So now we need to outfit everyone...
Pistol/Ammo - I vaguely remember in my time doing procurement for state government that we could buy a service pistol for ~$550 and training ammo (9mm) for about $150/thousand so that is $700/person
So let's say first year cost is $1700 for training and equipment. We have to pay everyone to go to training so let's say another $75/day x 4 days for $300. Let's assume we can avoid lodging and travel by paying the trainers to go out to regional sites (A spurious assumption, but I don't want to do the math) so all in we are at $2,000.
That would add a zero to the original estimate so training 3.3 million teachers would cost $6.6 Billion.
Of course this assumes that we feel good about 4 days of training being enough to adequately train these folks, and ignores all ongoing costs.