Genitive vs Ablative of Price

Question

In Latin, worth or value can be expressed by the genitive or by the ablative. Here are some examples:

Genitive

Non pono utrique par pretium: pluris aestimo beneficium quam iniuriam. (Sen Ep. Mor. 81.8.2)

Non ego te flocci facio; ne me territes. (Pl. Curc. 713)

Ablative

Metellus Scipio tricliniaria Babylonica sestertium octingentis milibus venisse iam tunc ponit in Catonis criminibus... (Plin. Nat. Hist. 8.196.8)

(Note: venisse is from venire (venum + ire): "to be sold")

...num ante tempus praemium petat et spem incertam certo venditet pretio (Cic. De Invent. 2.113.11)

My question: What is the rule, if any, for determining whether to use the ablative or the genitive?

Answer 1

Roundly, the ablative is used for price and the genitive for value.

The ablative of price occurs with verbs of acquiring, buying, selling etc., as in mensam quadraginta sestertiis emit. As well as specific forms there are, of course, general ablatives of price such as magno, parvo, vili.

The genitive of value (quanti, tanti, plurimi, nihili etc.), as you might expect, is used with verbs of valuing but does not express a definite price. A little confusingly, perhaps, such are also used with verbs of buying and selling — but never when expressing a definite price. Some examples, like nihili and often called vulgar in old dictionaries, compare the value to a worthless item, e.g flocci, nauci, pili (each of which you will recognise in the rather artificial English word floccinaucinihilipilification, " the act of valuing as worthless").