This is ripped off the druid wiki, i think it depends on the class?
Q: How do I know how much Mana Wisdom gives me?
A: As a druid you gain .21 more Mana per Wisdom(MpW for short) for every level you have when at or below 200 wisdom. This means that when you are level 10 you get 2.1 MpW and it continues on in a similar pattern for Wisdom totals of 200 or less: 4.2 MpW @ lvl 20, 6.3 MpW @ lvl 30, 8.4 MpW @ lvl 40, 10.5 MpW @ lvl 50, 12.6 MpW @ lvl 60. The exact formula for wisdom of 200 or less is (WIS+10)*level/5. The formula for calculating above Mana at more than 200 wisdom is ( [ 11250 * level ] + [ 33 * WIS * level ] ) / 425.
http://wiki.project1999.com/Druid
Down under the FAQ section