Can anyone tell me (or hazard a guess) at how D2 prestige compares to D3 prestige?

Is the lowest D2 prestige better than the highest D3 prestige?

If not, where do you think a A+ D3 prestige would rank among D2 prestige? 

Specifically, I'm curious where my A+ D3 prestige stands compared to a C prestige D2 school.

I would say far beneath;  I might view it as a continuum - It goes from bottom to top - lowest d3 to highest d3, above that lowest possible d2 is above highest possible d3, etcetera.

Well in the early days of HD there used to be a glitch that caused very high A+ D3 prestiges to show up as D- because they wrapped over into the D- D2 prestige range, so it's not impossible for a D3 team to have higher prestige than a D2 team.  Given that the relative recruiting value of prestige in both D3 and D2 is fairly small, I'd say most of your disadvantage against the D2 school comes from the money/scholarship difference.  If the player will talk to you you can definitely get him.  I've exchanged info after beating a D2 school in a battle a few times and always outspent them by around 10% to just over 20%.  It's not prohibitive with the number of open scholarships you have.