What numbers? You said that you wanted a guard at every door. It is an assumption that every school has 12 doors, but I can think of at least 12 exterior doors on the elementary school two blocks away from me. Frankly, that number is probably conservative when talking about some of the high schools I've seen in Texas.
So:
12 doors
1 security guard per door.
250,000 schools in the US
$50,000 a year salary for a security guard (that part is also certainly debatable, but I looked up security guard salaries for Ohio - 38,000 and California - 66,000, so 50K is probably an OK average guess).
12 X 250,000 x 50,000 = 150 billion.
I admit that this is an estimate, but they are reasonable numbers based upon the criteria that you set. They are not completely made up.
To my recollection, this is the fourth major mass shooting at a school (This one, Columbine, Parkland, Sandy Hook). Paul would think it is wasteful spending to drop 100+ billion dollars on paying for security guards at every school when this is, although horribly tragic, something that occurs extremely rarely. The guy literally proposed cutting the Department of Education of 83% of its funding in 2011. You think he would even remotely consider doubling it instead? I just don't see it.