Roman senators did not get paid. And they were certainly rich, but not necessarily the richest.
To be a senator, two requirements had to be met. Firstly, membership of the Senate was restricted to those people who had served in one of the higher magistracies (tribune, quaestor, praetor, or consul). Secondly, there was a minimum property requirement every senator had to meet. This property requirement was checked by the censors, if and when those were elected (nominally every five years, in practice very irregularly; during the 1st century BC there weren't any censors for decades). Censors also had the authority to kick people out of the Senate for "conduct unbecoming a senator" (to use a slight anachronism), but this almost never happened.
In short, Senators were rich people who had served in some sort of public function. To say if they were the richest or not, we'll have to look at non-senators for a bit. The same censors who checked property requirements of senators, checked the property of all Roman citizens and divided them into classes. The class for the richest Roman citizens was the equites class (usually translated into English as 'knights', but it's very different from the medieval knights people think of when they read 'knights' so many of us use equites or equestrians). The classes were initially based on who could serve in the military and in what role (everyone having to pay for their own equipment), but this no longer was the case after the end of the second century BC. Originally, equestrians were those rich enough to pay for their own horse and related equipment; they fought as cavalry if called upon. The exact property requirement was adjusted a few times during Rome's history. In general it's enough to say that any equestrian was vastly more wealthy than any ordinary Roman.
So there's this equites class which had a minimum property requirement, but how rich could they be at the upper end? Filthy rich, that's how rich - because there is no property class above equestrian! In simplified descriptions you'll often find that equites were below senators in property class, but this is not true at all. As we have seen, the senators weren't just a property class, there was another requirement. If an equestrian amassed a huge wealth but never bothered to perform a public function (Titus Pomponius Atticus for example), he would never become a senator.
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