David, you are exactly right: tantum is being used here substantively; like many words in Latin, it can function as an adjective or as a noun. We do the same in English: "I can only take so much!" (nominal) ... or ... "Latin pronouns cause me so much trouble!" (adjectival). What's tricky here is that in addition to the nominal use of the neuter adjective, you also have an idiomatic use of the genitive. It is sometimes call partitive genitive - we say "so much money" in English, but in Latin you say "so much (of) money," tantum pecuniae.
Just to brace yourself for even more confusion later, the neuter tantum is also commonly used as an adverb in Latin (many Latin adverbs are neuter adjectives).
So, depending on context, tantum can be an adjective, or a noun, or an adverb.
The HUGE article about it in the Lewis & Short dictonary alerts you to the wide range of uses of this word and also lets you know that is one of the most commonly used Latin words. Here is the dictionary entry:http://athirdway.com/glossa/?s=tantus