Although Daphne is depicted as being the wealthy member of the Scooby Gang, Fred may be just as wealthy. Signs of Fred's wealth appeared at the very beginning of the Scooby franchise.

First, there is the fact that Fred wears an ascot in Scooby Doo Where Are You!

Here is an explanation of the ascot's history from Quora:

That ascots "are worn mainly by yachtsmen, jetsetters, or those trying to portray an aristocratic attitude" is seen in the 1960s American TV show Gilligan's Island in which millionaire Thurston Howell III is often seen wearing an ascot.

Second, there is the technology that the Mystery Machine is equipped with. For example, here is a scene from Decoy for a Dognapper:

The Mystery Machine itself is a 1966 Chevy van, meaning that the van was only 3 years old at the time that the Gang is first seen using it.

Third, there is the boat that Fred uses early in Scooby Doo Where Are You!

Plus, there is the diving equipment that the boat is equipped with.

In short, Fred had what were expensive toys back in the 1960s.