nisi jam dudum gaudimonio dissilio, quod te talem video

 jam dudum: Also written as jamdudum or jandudum, now after all this time. [dudum OLD 3]

gaudimonio: As noted by the commentary, the suffix –monium is simply a Vulgar enlargement of the –ium ending. It may have been derived from adjectives that used to end in –mo, -monis.

dissilio: To burst apart. [OLD 2] This verb is more commonly used in poetry (such as by Lucretius, Vergil and Ovid). It has been suggested by R. B. Steele that Petronius may be modelling some of his phraseology after Seneca. In his Dialogues, de Proventia 4.10, he uses the similar words, cruditate dissiliunt (they burst apart from excess food). Seneca wrote condemning such vices as overly excessive banquets and this may be Petronius’ way of satirizing Seneca. Especially given that he has just written in section 39.3, oportet etiam inter cenandum philologiam nosse.

te talem: Niceros is addressing Trimalchio. talis in apposition can refer to the quality/calibre of an individual or that one is an exceptionally good/bad type of character. [OLD 3b] Here possibly referring to the geniality or friendliness of Trimalchio, since in a preceding clause Niceros is delectatus affabilitate amici.

quod: Used as a conjunction.

ut mentiar, nullius patrimonium tanti facio

The purpose of this exaggerated statement of Niceros is to convince his fellow diners that he could not possibly be lying.

patrimonium: A patrimonium is property, including possessions, estates and money, which belongs to the paterfamilias. By extension it also means inheritance. The point of using this noun here is to refer to a large quantity of wealth.

Petronius has already used this noun with a similar sense in section 44:
plus in die nummorum accipit, quam alter patrimonium habet
(he receives more money in a day than another man has in inherited property)

facio: The verb facio can have the sense of to regard/consider/treat something (as). [OLD 18b] When used in conjunction with the genitive case it can mean to esteem or value (at). [OLD 18c]

nullius tanti: Genitives of quality which denote indefinite value.

ut mentiar: A result clause, indicated by tanti.

In English one might say the similar sentiment of, “I wouldn’t lie for all the money in the world.”