Great grounding perspective! Personally, if my son does get drafted out of high school, my assumption at this point is he wouldn't go high enough for what I would consider life changing money. Although my threshold for life changing may be higher than others. But I am a very analytical person by nature and love to crunch numbers! Not an exciting hobby! So at some point, I would take a stab at what is the magic number just for the hell of it. So all perspectives, advice and your guidance are greatly appreciated!
IMHO, I feel "life changing money) for a draftee out of HS with family of average income would be amounts you see for 2nd rounders. . . and keeping in mind that the federal government will want about 36% and depending on what state(s) are involved, it can easily be a total taking better than 50% of these bonuses. And for HSer, negotiations should include paying for 4 years of college (amount equal to amounts for the colleges that player would go to if they didn't sign).
Now, there are many players coming out of HS that really have no interest in going to school (they may change their mind about that at some later date as they mature). Some don't have the grades to get into college. So, "life changing" would likely be something less, like money one would see as low as the 5th round.
You can crunch numbers that are more specific to your son and one needs to keep in mind that if your number is is viewed too high for what the scouts see as their potential, then he may be viewed an "not signable" and therefore not picked as high in the draft as one might have expected.