The first problem in my opinion is the "power" of the spell. There are two sides to this, really. First of all, the internal competition in the dwarven spellbook is very strong. Beorn is a beast capable of deciding smaller fights on his own, and everybody knows that towers are awesome. Greed, on the other hand, gives you money. Sure, more money is always great, but the dwarves don't profit from an income boost any more than other factions. You could even argue that most of them have some way of using 1500 extra cash better (Mordor is very dependant on heroes, Gondor has more buildplots to get additional economy buildings, Loriens base building is very expensive), the obvious example being Isengard. They have the devastation(?) spell that looks quite similar, but is in my opinion stronger just because of working under different circumstances. Devastation allows Isengard to buy scouts and wildmen as much as possible, because it can pay for teching up by itself. You get something like 1500+ resources, that's enough for getting a second uruk pit and teching up in the first one. And tadaa, you suddenly have two buildings you can spam uruks with.
Tl;dr^^: The dwarves can usually get better use out of their great combat spells than economy spells.
That doesn't mean that it is worthless, though. It is just a bit more niche than the others. Which leads me to the other problem I have with greed: Three points for a somewhat decent chance at getting 1000 resources? No, thanks. So the first, most important step has to be increasing the reward part of the risk-reward-system^^
Adrigabbro's idea seems fine to me, but I would change the values a bit. Something like 50%-25%-15%-10%. 75% chance of success in total is still not that great, and the possibility of RNG not working in your favor is very real. Wasting three spellpoints and getting nothing for it is huge. Maybe the summoned goblins could drop increased exp/spellpoints, so at least you get something for killing them. You could probably even increase the money you get, but the important thing is that the overall income is worth the three spellpoints.
At the moment, it is not.