What is "fairly substantial" here? $10k? $20k? $100k? I certainly know people willing to risk the first two but not the third (primarily early 30s professionals

What is "fairly substantial" here? $10k? $20k? $100k? I certainly know people willing to risk the first two but not the third (primarily early 30s professionals). $20k is plenty to participate in the seed round of a software startup. It seems silly to forbid people to invest an amount smaller than they would spend on a car (a guaranteed-depreciating asset over the next 10 years).