As a spellsinger, my mp regen went from 21-22 (NM set + buffs with -8 men) to 26-27ish with song of Meditation... which is pretty decent. Thats roughly what I get from ESs lvl3 mp regen pony buff.
Its hard to tell exactly how much a skill costs what with mp regen making it hard... so I dunno how much it lowers mp use by.
The difference was noticeable with and without meditation for me anways.
if you have renewal/meditation and do anything with a mage group the change in mp is significant. you regen more, use less mp per spell and on top of that reuse-timers of spells are shortened. they might sound like they have little benefit, but in reality its a very strong combo which makes a huge difference for any full mage group and is worthy to use under certain circumstances in other group setups, too.
MP regen rate is increased 30% (maybe 25%) with Meditation
MP Usage is lowered by roughly 10% from Meditation and roughly 5% from Renewal. Renewal lowers MP use for BOTH physical and magic whereas Meditation ONLY lowere MP usage on magic.
Renewal also significantly lowers Re-use timer on BOTH magic and physical.
Champion (lvl 78 song) is said to lower MP usage by 20% on physical ONLY and it lowers Re-use timer on physical ONLY.