Are “genitives of value” the grammar term for how one (always) turns a quantity into an expense? And otherwise the quantity means size?
I guessed that Tanta….quanta meant “as much as”; literally I got “such an amount….so much”
The neuter forms
tantum... quantum used substantively (i.e. as if they were nouns themselves rather than in agreement with another noun) mean "as much as" (literally "as much... how much"). For example:
Tantum edi quantum volui = "I ate as much as I wanted". When these same forms are put in the genitive, but again without modifying a noun, they convey price or value. E.g.
Tanti domum vendidi quanti volui = "I sold the house for as much as I wanted";
Haec domus tanti est quanti illa = "This house is worth as much as that one (a bit more literally: is of as much [value] as that one)".
When the corresponding adjective forms are used as regular adjectives in agreement with a noun, like
tanta and
quanta in your example, then the translation more often involves words like "large", "big" or "great" rather than "much" (though there are exceptions).
I thought that quam was used for “as” in comparisons; e.g. tanta…quam “such an amount as”. Is quam not used in this way?
Quam is used for "as" principally when the correlative is
tam. E.g.
Marcus tam fortis est quam Gaius = "Marcus is as brave as Gaius."
The correlative to
tantus (and all its forms) is usually
quantus (and all its forms) rather than
quam. Similarly, the correlative to
tot is
quot (
tot... quot = "as many... as"). Yet another example of a common correlative pair is
talis... qualis = "such... as", "of the same kind... as". In sum, the English "as" in the second part of the comparison can translate in many different ways (
quam,
quantus,
quot,
qualis, etc.), depending on what comes before it (whether it's "as much" or "as many" or "such" etc.).
