I think the multiplier should be exponential instead of linear. As in, a T1 mob isn't just a 'bit' more difficult than a T0 mob, and a T2 mob isn't just the same 'bit' more difficult than a T1 mob.
That being said, I would think that the base gold given to a mob should be reduced a bit slightly (T0 mobs are really pushovers, even level 120 T0 mobs given that you have decent speed), and the formulae could be something like a + a*(t^(a/b)).
a - gold given for a particular level mob
t - tier level
a/b - a suitable ratio which lies between 1 and 2, ie, a > b && a < 2b.
But would the above equation be feasible only if the number of T0, T1, T2, T3 and T4 mobs are equal? Ie, in the entire game, there are let's say, 1000 T0 mobs, 1000 T1 mobs, 1000 T2 mobs, 1000 T3 mobs, and 1000 T4 mobs. Then the gold given by the higher level mobs would help subsidize healing potions / spellup potions used to kill them.
In the game, the majority of mobs are T0s and T1s in most areas, and the gold given by them would (somehow), balance the low gold given by the T2, T3, and T4 mobs. I would assume that a normal player (well, maybe just me), just levels in areas with many T0, T1 mobs (xp gain faster, only if the number of T0, T1 mobs > T2,3,4 mobs in a similar area), and uses the gold to go occasionally against a higher tier mob if it is a quest mob / holds decent eq.
In fact...I..er...haven't ran out of money when fighting higher level tier mobs, but instead have increased the amount in my bank account. So I'd say keep the equation as it is, or change it but reduce the amount of gold gained per level, or just reduce the amount of gold gained per level.
