Getting back to the original question:
As a rough estimate, plan on pocketing about 5/8 of the bonus money after state and federal income taxes. This will vary depending on how big the bonus is (and therefore, what your tax bracket will be), but surprisingly, not by that much.
Talk to an investment advisor about how much annual income you could expect that remaining principal to throw off. Then figure out how much you'll need to live on. Bear in mind that in rookie ball, the pay is about $1,200/month gross, and you only get paid during the months you're in uniform. You'll probably earn $7,200/year, and with taxes and a housing/food allowance taken out, you'll not have enough cash to make ends meet. That means either you continue to live off your parents (as you would if you went to college, most likely), or you work in the offseason, or you start eating into that bonus money.
If you asked 1,000 people how much it would take to get them to go pro out of HS, you'd get 1,000 different answers. It does depend on how much you value a college education and the life experience of college (which, let's face it, is one of the most fun times of your life). It will most likely also depend on your background. If you come from money, a low six-figure bonus may not turn your head; you probably look at yourself as someone who can pull that down in the future anyway. If you come from a modest background, where retirement is often equated with trying to live off Social Security alone, low six figures sounds like an unimaginable amount of money, instant riches.
Most HS draft signees get 5-figure bonuses, and their money is gone within a few years. And most don't make it to MLB. So they can end up 25, broke, uneducated and unemployed. It's a cautionary tale, but it gets repeated all the time. On the other hand, there are plenty of young adults with no college and no savings, and lots of them start new careers and do just fine. It's just a lot harder to pull off than is life with a college degree on your resume.
A low six-figures bonus can also evaporate much more quickly than many think. Especially if you're the type who pays cash for a new car the moment the check is in your bank account.
Most kids would find mid-six figures hard to pass up, but every year there are many who do, and this year will be no exception. That's a tough call, because if you are in that bracket, you have to face the proverbial "bird in the hand/two in the bush" dilemma.
I can't imagine anyone turning down 7 figures, though I suppose someone somewhere would. But then, only about 15 high school kids per year even get that option. Not exactly a statistically significant percentage of the kids out there dreaming the big league dream, eh?
For most of these kids, the difference between what they can get out of HS, and what they would get if they waited until after college, ain't all that much. Often you can even improve your stock and do better in the draft after college. The mistake so many kids make is in thinking that it's an either/or -- either pro or college. The reality is, you can have college now and pro later, and not that much later at that. (Ask TPM how quickly it goes by!)
IMHO, a lot of these kids taking checks at age 18 would have been well advised to get college done and come back to their pro dreams at age 21 or 22. If they truly have the talent and the drive to make it in the pros, the option will still be there at that age. And if the option is gone due to injury or failure, then better to have found that out while getting a degree than not.