According to Plutarch:

People were wont to say that the many virtues of Crassus were darkened by the one vice of avarice, and indeed he seemed to have no other but that; for it being the most predominant, obscured others to which he was inclined. The arguments in proof of his avarice were the vastness of his estate, and the manner of raising it; for whereas at first he was not worth above three hundred talents, yet, though in the course of his political life he dedicated the tenth of all he had to Hercules, and feasted the people, and gave to every citizen corn enough to serve him three months, upon casting up his accounts, before he went upon his Parthian expedition, he found his possessions to amount to seven thousand one hundred talents; most of which, if we may scandal him with a truth, he got by fire and rapine, making his advantages of the public calamities. For when Sylla seized the city, and exposed to sale the goods of those that he had caused to be slain, accounting them booty and spoils, and, indeed, calling them so too, and was desirous of making as many, and as eminent men as he could, partakers in the crime, Crassus never was the man that refused to accept, or give money for them. Moreover observing how extremely subject the city was to fire, and falling down of houses, by reason of their height and their standing so near together, he bought slaves that were builders and architects, and when he had collected these to the number of more than five hundred, he made it his practice to buy houses that were on fire, and those in the neighborhood, which, in the immediate danger and uncertainty, the proprietors were willing to part with for little, or nothing; so that the greatest part of Rome, at one time or other, came into his hands. Yet for all he had so many workmen, he never built anything but his own house, and used to say that those that were addicted to building would undo themselves soon enough without the help of other enemies. And though he had many silver mines, and much valuable land, and laborers to work in it, yet all this was nothing in Comparison of his slaves, such a number and variety did he possess of excellent readers, amanuenses, silversmiths, stewards, and table-waiters, whose instruction he always attended to himself, superintending in person while they learned, and teaching them himself, accounting it the main duty of a master to look over the servants, that are, indeed, the living tools of housekeeping; and in this, indeed, he was in the right, in thinking, that is, as he used to say, that servants ought to look after all other things, and the master after them. For economy, which in things inanimate is but money-making when exercised over men becomes policy. But it was surely a mistaken judgment, when he said no man was to be accounted rich that could not maintain an army at his own cost and charges, for war, as Archidamus well observed, is not fed at a fixed allowance, so that there is no saying what wealth suffices for it, and certainly it was one very far removed from that of Marius; for when he had distributed fourteen acres of land a man, and understood that some desired more, "God forbid," said he, "that any Roman should think that too little which is enough to keep him alive and well."

Source

At his death it is said that he was worth 170 million Seterces, equal to the entire annual budget of the Roman Empire. Imagine if Bill Gates was worth 3.03 Trillion dollars, which was the US Budget for 2014.

This is a surprisingly common question. Here is my answer from another thread on the same subject. The short version? Not really that rich. Crassus, although Plutarch evaluates him at 200 million sesterces, simply didn't have access to the big money-makers of Roman society--tribute and a province. 200 million sesterces was what Rhodes paid to Cassius to keep him from sacking the city, which was a fraction of their wealth. Considering that people like Pompey had cities like that and richer paying them tribute Crassus' wealth was practically insignificant