Do you have a choice in which insurance plan to use? Sometimes there's a cheaper plan with similar benefits, or might be cheaper for your number of employed...remember sometimes the family plan covers a LOT of people, others are sometimes broken down as self, self+one, self+three, etc...worth looking into.
Reason the family plans get so expensive is that employers usually pickup a large(r) portion of the first person on the policy, but pay less/none towards the additional insured people (so you windup paying a larger share out of pocket).
For the economics question: assuming you're 35yo now, when you were 18yo (1991), minimum wage (federal, some states were higher) was $4.25/hour...which translates into about $6.70/hour cents today once you factor in inflation.
2,080 hours a year working full time (40hrs/wk) = gross income of $13,936 in today's dollars. You were supporting one person, yourself, off that money; paid little or no income tax, and possibly/likely had additional support from family.
Today, you're supporting a family of 3 (or 4, wasn't clear) off some level of income where $500/mo of pre-tax dollars is a large burden. 1-to-1 multipliers are not very accurate, but while your income surely had gone up anywhere from 3-5 times (reasonable incomes where $500/mo is a pain) your 18yo income, your expenses undoubtedly have kept pace or increased, relatively speaking...a la the more you make, the more you spent school of thought.
Some of these increased expenses were your choices (children, where you rent/own, having a car or not, if you eat better/healthier now than at age 18, etc.), others were imposed, at some level, on you (higher fuel costs, higher insurance costs, etc.) and others were outside your control (randomly imposed costs like a car accident). All in all, the sum of these choices more than likely mean your disposable income might be lower than at age 18, but that your standard of living is higher. Choices...
Back to insurance, if there is a spouse, does he/she have health insurance that is cheaper for kids to be on? Can you qualify for a hardship exception to income limits for your state insurance for children? Another option may to look for another job (tough job market sadly) with better health benefits. Good luck to you.