In The Great Gatsby, what does Nick mean when he says "They’re a rotten crowd...You’re worth the whole damn bunch put together.” ?

In Chapter Eight of F. Scott Fitzgerald’s novel
The Great Gatsby, Gatsby reveals to Nick the full extent of
his history with respect to Daisy Buchanan, his one true love – or obsession.  She was,
he tells Nick, “the first ‘nice’ girl he had ever known.”  Gatsby’s problem, of course,
was that he came from a very modest background and could not hope to compete for Daisy’s
hand in her world of affluence and conspicuous consumption.  She was aristocracy; he was
proletariat.  As he describes the context under which he finally consummated his love
for Daisy, he reveals more about his background:

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“However glorious might be his future as Jay
Gatsby, he was at present a penniless young man without a past, and at any moment the
invisible cloak of his uniform might slip from his shoulders. So he made the most of his
time. He took what he could get, ravenously and unscrupulously—eventually he took Daisy
one still October night, took her because he had no real right to touch her
hand.”

Gatsby, unscrupulous
though he be, knew he wouldn’t measure up socially and financially to Daisy’s world. 
That, however, didn’t stop him from committing his life to the accumulation of wealth so
that he could at least position himself geographically and emotionally closer to the
object of his obsession.  He knows, however, that the gulf between them is
insurmountable.  It is Nick, however, shorn of his idealism and naivete, who assures his
neighbor and “friend” that wealth and class are two very different things.  Talking with
Gatsby late in this chapter, the two are preparing to go their separate ways, Nick
preparing to catch a train to the city.  Assuring Gatsby that he’ll “call you about
noon,” Gatsby suggests that “‘I suppose Daisy’ll call too,’ to which Nick responds, “‘I
suppose so.’” Nick then describes the scene as
follows:

“We
shook hands and I started away. Just before I reached the hedge I remembered something
and turned around. ‘They’re a rotten crowd,’ I shouted across the lawn. ‘You’re worth
the whole damn bunch put together.’

“I’ve always been glad
I said that. It was the only compliment I ever gave him, because I disapproved of him
from beginning to end.”

Nick
is pointing out what the reader has been able to see throughout The Great
Gatsby:  The wealth and status enjoyed by the Buchanans and those with
whom they associated was a thin veneer beneath which was a foundation of moral decay. 
Character is more important than money, and Gatsby, if nothing else, had
character.