How much of a stat bonus is each revocation of my characters worth? Does it depend on which class I'm revocating?
Revocation does not change stat growth - that is still entirely based off the character's Vocation and level. Revocation simply reduces that character's level in the Vocation back to level 1, allowing them to earn another 200 skill points to be allocated as necessary. One complete 1-99 revocation is necessary for a character to master all weapons and vocational skills. (2600 skill points needed for max mastery vs 12 vocations at 200 points from levels 1-99.)
Additionally, revocating your Hero has an affect on the quality of Grotto Maps that you find once completing a Grotto. Far more information than I can fit into this text box can be found on that here.