I just wanted to make it clear that I wasn't taking any position on passing on draft $$$, just pointing out that it is easy for us to talk about $500K after taxes when it's hypothetical. It's a little more complicated in the real world. This decision is a very, very, circumstantial one in which several factors must be considered:
1) What school are you committed to?
Stanford and Akron are not equal.
2) What is your family's financial situation?
Again, Vandy from a family that can help support you for three years is not the same as coming from poverty with a 30% scholarship from Northern Colorado
3) Are you ready to play professional ball?
Emotionally, this is a big step into the real world for an 18yo. Are you emotionally ready and mature enough to step into the brutal realities of low-level minor league ball that is a business? A world where the real competition isn't on the field, but rather is the guy sitting next to you in the bullpen?
4) What are the chances you will improve your draft status in three years?
This may be THE factor when talking money in the high-six-to-seven-figure range. If you're a big-time talent headed to a Power 5 school and don't feel like you've fully developed yet, this could be a big time, low risk gamble that could turn a million dollar bonus into 5 million if you're willing to wait.
My 2018 turned down money from two clubs wanting to draft (nowhere near the money we're talking about - NOWHERE near). However, if he entered pro ball now, as a young 18yo, he'd most likely be pigeon-hold into a reliever slot. He needs more work and development. What he doesn't need is years of bus travel and bad living conditions. However, he's not going to Vandy or Stanford, so, almost no matter how he develops in college, he's not coming out of it as a top half of the first round guy. For him, a third round slot would have been something with enough money that he wouldn't have gambled by passing it up.