Yes, 100k per year is a good income. You can basically have a one-worker household with several kids on that most anywhere, and still have some money left over. If you wanna send your kids to private school or buy a $700k house though, you'll run short quick.
At the same time, the US isn't "super cheap". Places like Eastern Europe and Beanerland are "super cheap". There are "normally" cheap places in the US where you can buy a big house for like $100k but they have very little industry and it's hard to make money there in the first place. Earning $60k/yr. in a place like that usually means you're making double the median household wage.
Finally, $40k/yr. isn't shit. In depressed areas it's certainly "enough" for a single person, but the budget quickly runs dry. You might take home $34k or so after taxes. $2k per month on mortgage, utilities, internet, food, and gas... maybe a little bit left over for a night out or two. Health insurance might cut into this, even good insurance is about $150/mo. for a single. That leaves you $10k at the outside for the year. Car insurance and other necessary spends, household goods, shoes, clothes, and so on cut into this. Need to buy ONE appliance or a decent lawnmower or something? That's a month's savings. Once you pay for big stuff and emergencies, there's not much left over. What are you gonna save, 5-6 grand a year if you live tight and don't have any bad luck? That ain't shit. That can disappear overnight if something blows up.
Anyways I dunno exactly how you're indulging your hobbies or if you live in a shack, but personally I'm not partial to buying junk cars for my lawn. Going to the track a few times a year might cost $3-4k once everything is said and done, and unless you're running a Miata those numbers are impossible to get down. Clean sports car will cost you 20k alone. It just doesn't add up. Forget having kids on that income too.