I am trying to figure out how big an advantage prestige is when recruiting. I have a C prestige at a Big 6 Conference school. How far behind am I if I battle for a recruit against a B prestige school? (If Distance is irrelevant) How about an A prestige school? Do I have basically no shot against a team with A Prestige?

Significant.

I always need to look it up when recruiting in D1. Search prestige advantage or something similar and you can find some significant discussions on it.

According to losymyth's D1 guide, its a 70% advantage per letter grade.  This was written over 4 years ago though, anyone know how accurate this info still is?

the 70% thing is tough to understand as a standalone stat.

IMO it's better to think of it in dollar amounts.

If 2 teams of equal distance - one with a B and one with a C (lets ignore high or low B/C grades) are going after the same player - how much more cash is the C team going to have to spend to equal the B team?

If we use the 70 percent rule for each grade level - that means if the B team spends 10k, the C team would have to spend 17k to have equal weight. Right?

your math is correct, but I think it is wrong to ignore high or low B/C grades. 

Since prestige is a letter grade representation for a range of numeric values it is risky to assume that any C is exactly 1 letter grade below any B. When determining battle values, IMHO it is wise to assume you are the lowest possible whatever grade (C in this case) and your foe is the highest (B). If you are .01 ticks or whatever over the line of C/C- and your opponent is similarly close to B+ your real difference is much closer to 1 2/3 grades, meaning if you assumed you were safe with that 17k (for example) but really to equal his 10k you would need  21,600...

Also remember that A+ seems to have no upper limit, meaning that if you hit A+ at 97 of 100 or whatever it seems possible to go to well over 100 (or whatever the values are...) so A+ is even more volatile.

To your second question OP, you always have a shot, depending on circumstances. Most of the time you won't be able to take A schools, but at the C level its unlikely you'll be recruiting the same players unless said A got poached and is trolling for scraps...