Something else that might impact things:

Scooby Doo MAY be set in a period of economic recession.

It is never outright said in the show or by any of the creators, but there is a lot of evidence that points towards the Scooby Doo universe being in the middle of an economic recession:

(source: http://scoobyfiles.toonzone.net/monsters/index.html, as well as a number of other articles that pop up when googling "Scooby Doo recession")

1. Nearly every episode was set in some sort of abandoned place where there would otherwise be a lot of people: an airport, mansions, castles, a ski resort, an amusement park,... Every location looks broken down, unused and abandoned, like the economy just doesn't support the activities they encompass.
2. Nearly every criminal was motivated by monetary greed: a rare coin, a family treasure, a smuggling operation, theft for resale,... And even those that weren't motivated directly by monetary greed just wanted a job, or in one case were motivated by hatred against robots, who tend to steal jobs. There were no crimes of passion or crimes of hatred of other people, which means that most people just wanted to get money.
3. Most of the criminals had skills that could easily land them a job: several PhD s, lawyers, engineers, scientists, entrepreneurs,... People that could easily legitimately make money using their skills in legal manners,... If there were any jobs around.
4. They don't seem to make all that much money for food and other necessities. In a number of episodes, Scooby and Shaggy react very hungrily when they encounter large amounts of food, as if they don't know when they next get food. And they seem to easily be bribed by relatively small amounts of Scooby Snacks from time to time. And look at how scraggly Shaggy looks. It seems like they're struggling to get by for the most part.
5. The gang actually is around high-school age: between 15 and 17. They might look older, but their official age as given in the show bible were given as Fred and Shaggy being 17, Daphne being 16 and Velma being 15. So if these kids are of high-school age, why are they not at school? It seems like these are just a bunch of bored kids that have nothing better to do with their time than to drive around the country in search of things to do.

All of this evidence points to Scooby Doo, and especially the original 2 seasons, to be set in the middle of an economic recession. Most people with specialized skills don't have jobs, and have turned to crime in hopes of getting money. Tourism and education have gone down the drain because people don't have the money to travel or pay for education.

So they might simply not need that much money because there isn't that much to spend it on. The gang may just simply be a roaming group of high school age teenagers that are in search of anything to do.