Crassus, who even in antiquity had the misplaced reputation of being the richest man in history, is assessed by Plutarch as having 7100 talents on the eve of his Parthian expedition, a sum slightly less than the 8000 talents that Cassius extorted from Rhodes when he captured the city, by confiscating all the gold and silver in the city. Caesar's debt before attaining the quaestorship was thought to be crushing and enormous--Plutarch gives a figure of 1300 talents. To give a somewhat closer sum, Roscius claimed that his father's estate, confiscated in Sulla's proscriptions, had been worth 250 talents. So Cato's share of his inheritance was not an enormous amount (although I will say here that the Greek is a little bit unclear--it may mean that the 120 talents were from the sale of the house left to him, or it might mean that he moved his residence after receiving his inheritance--the second I think is more likely, since Cato lived most of his youth with his uncle Marcus Drusus), but it was nothing to sneeze at. That was hardly Cato's entire fortune, though, he had quite a lot more money than that and by the middle of his career was surely just as wealthy as any other member of the nobiles could be expected. When his brother Caepio died he inherited his fortune, and we know that he held lavish funeral games at his own expense, and he was financially secure enough to distribute an inheritance from his cousin of 100 talents to any of his friends who needed the cash. Shortly after this episode Plutarch tells us that Cato donated a fair amount of wealth to the public treasury:

ἔνιοι δὲ καὶ χωρία καὶ θεράποντας αὐτοῦ διδόντος καὶ βεβαιοῦντος ὑπέθεντο πρὸς τὸ δημόσιον.

But some [of his friends] pledged both land and slaves to the public treasury with him offering, and he made good his offer

This additional donation cannot have been a small sum of money, since slaves and property (by which we mean estates donated to the ager publicus) were not cheap) and it certainly didn't leave Cato penniless. Cato also played the game of dowries and inheritance through his fathers-in-law. It's not known how wealthy his first wife Atilia was, but he tried to marry Aemilia Lepida before Atilia, who was of an extremely prestigious and wealthy family--the engagement was broken off because her former betrothed, Metellus Scipio, decided he wanted her back and Cato was apparently very upset. But famously his marriage with Marcia was considered an attempt at increasing his own fortune. Cato divorced Marcia temporarily and married her off to Hortensius, the famous orator that Cicero had curb-stomped in the in Verrem. Hortensius was certainly fabulously wealthy--Cicero considered him and Lucullus the exemplars of the sort of lazy, wealth-corrupted nobiles that were content with wasting their time and effort cultivating their lavishly expensive fish ponds, giving them the insulting moniker the piscinarii, "fish-fanciers." Hortensius was an old man, and when he died a few years later Marcia inherited everything he had and promptly re-married Cato. The episode is shrouded with mystery, but even if we don't accept Caesar's accusation that Cato was dealing in wife-trafficking it's clear that by this time at least, and surely long before, Cato was stupidly rich. Even before this he helped provide the enormous bribes during the consular elections for 59 to get Bibulus elected as Caesar's colleague--that election was noted for the extraordinary sums of money spent by both sides, and Cato claimed that in this particular case it was perfectly fine to bribe people (which amuses me about as much as his justification for marrying Marcia off to Hortensius despite having divorced Atilia for infidelity). Cato was also the great-grandson of Cato the Elder, who held hugely expensive estates all over Italy--it's very hard to believe that somehow the family, which was considered to take after Cato the Elder in their frugality, had managed to squander it all away in only a few generations

A few hundred years prior Alcibiades married Hipparete for a dowry of 20 talents. That sum was considered huge at the time. I think the total annual revenue of Athens around 400 bc was something like 1000 talents.

A talent is actually a unit of weight measurement which is roughly 70 or 80 pounds. Assuming we're talking about gold talents, of which Cato got 120, you could say that he received something like 9,000 pounds of gold, or it's equivalent value.

This would've been a large sum and I'd certainly feel honored if anyone would like to leave me such an amount in their will. However, it's hardly an extravagant sum for the times. For example, when Caesar was captured by pirates they originally demanded 20 silver talents for his ransom, when he was still just a private citizen of a high social class, which might give you some idea of what 120 talents could buy you, although I stress that Caesar's ransom was silver talents and I'm not sure if Cato's inheritance is reckoned in gold or silver talents.