You only get money for the first 10 rounds though and if you pay anyone from round 11+ more than 100k, it goes from your pool. So the only way you can pull it off is if you sign people from first 10 rounds to below slot value.
If it was possible to offer players up to 500k, they would be more likely to sign than go to college. Those that are already at college will probably sign because they know if at college they were drafted so low, it won't be any better next year.