Linguistically, they are the same word but, as often in Latin, the neuter singular form can take on a life of its own.
quantum pretium: quantum here is an adjective modifying the noun, "what price?" "how great price?"
quantum pecuniae: here pecuniae is in the genitive, and the word quantum is being used on its own as a neuter noun, not as an adjective: "how much (of) money?" - the answer could be: multum pecuniae! a lot of money!
Like quantus, multus is an adjective where the neuter form - multum - is often used on its own with a partitive genitive, and it can also be used adverbially. If you look in a dictionary like Lewis & Short, you will see that the entry both for quantus and also for multus have sections devoted to the special uses of the neuter singular forms; this is true for lots of Latin words where the neuter singular has developed its own special uses:http://athirdway.com/glossa/?s=quantushttp://athirdway.com/glossa/?s=multus