I think $6 million might have been a little low but not necessarily.

In her new book, Victoria Gotti claims that she doesn't know how much her father was worth but thinks , based on later conversations with her brother that it was somewhere around $12 million. In Selwyn Raab's book , FBI agent Bruce Mouw thought that Gotti was worth at least $20 million before his final imprisonment.

As the series continually pointed out, Tony came in at the end of a cycle. He also wasn't the best money maker in the world. His Family was relatively small and would have had to share New Jersey not only with the NY family(ies) but also with Philly.

Also illegal gambling has to compete with state lotteries; loansharking has to compete with payday loans and credit cards and even executive gambling games have to compete with nearby AC casinos. And of course everyone who works for Tony is a criminal so they will never kick up every piece of tribute that they're supposed to.

So $6-8 million net worth may not be that far out of line for a relatively small time boss in the modern era.