You get about half as much mana as you would.
To explain let's say you had a Hume, whm at lvl 10. You would have 85 mp. Now let's say you sub that for your 20 war, now you have a 20 war / 10 whm, and would have 42 mp.
The numbers are from here:
http://ccwf.cc.utexas.edu/~nhart/charstat1.html