He's not insulted. His "Aw, you're telling me...." is more of an "I don't believe it, it's too good to be true, you're kidding me" kind of a response. Not a response of being insulted.

That is why The Turk says, "Sonny was hot for my deal" because the Turk, seeing Sonny's reactive outburst realized that Sonny was sort of awed that the would be guarenteeing such a large investment.

Sonny's response made the Turk realize that he had gotten Sonny's attention. So the Turk's attempt to get Vito out of the way was his way of responding to Sonny's outburst, saying "I have you interested now huh? Yep I'll guarentee the investment. Unbelievable deal, right? A can't lose proposition."

Sonny's response showed interest in Sollozo's offer. He was not insulted.