AFAIK if both the global spell and the Dispel pay the base cost, there is about a 50% dispel chance. This is however modified, depending upon (for both spells) :
- extra gems spent,
- difference in skill of the caster and the level of the spell.
There is also an open-ended d6 added to the gem input of every global and dispelling attempt.
Exemple : a lvl 10 nature mage casts Gift of Health (lvl 5) with 25 extra gems. Sum=25+5+D6(oe). Dispelling astrologer has astral lvl 3 only but uses 35 extra gems. Sum=35+D6(oe). Dispel will probably succeed (35+d6 vs 30+d6) : it's hard to beat a difference of 5 with one (even open-ended) d6.
Cheers