Mana regeneration in classic WoW is the mana Spirit returns every two seconds outside the five-second rule; it is 13 + Spirit / 4 for a Priest or Mage.
mana per 2-second tick, out of the five-second rule:
Priest, Mage: 13 + Spirit / 4
Warlock: 8 + Spirit / 4
Druid, Shaman, Paladin, Hunter: 15 + Spirit / 5
"mana per 5" from gear = that amount every 5 seconds, always
Spirit regenerates mana on the two-second tick; multiply by 2.5 to compare with "mana per 5". Each class has its own line, so the same Spirit is worth more to a Priest than to a Druid.
| Mage with 300 Spirit | 13 + 75 = 88 per tick = 220 per 5 seconds |
| Druid with 300 Spirit | 15 + 60 = 75 per tick = 187.5 per 5 seconds |
| Priest with 300 Spirit over a 3-minute fight, casting without pause | 0 from Spirit (see the five-second rule); only the gear's mana per 5 counts |
Last checked: unverified against Forever. These are the long-established values of the classic ruleset (the 1.x game of 2005-2006); World of Warcraft Forever is in beta and may differ. When you test this one in Forever, post your numbers in Theorycraft.