I've seen that quite a few people set their Advanced Scouting budget at $0 and then make their own judgements based on past player progressions. To a degree, I get that.

But how do that players have any idea how a rookie or international will pan out? Wondering how confident they can be making trades, too.

Rookie projections are based on your draft scouting budget. Internationals on your International budget. Advanced scouting gives you projections of other teams' rostered players, and your own players after you acquire them. If you have 0 Advanced, the projections you see will be useless - they're usually sizable overprojections. So you just have to remember (or record) what the initial projections from the higher budget were.

Trading is another thing and I've addressed it in the other thread.

Let's say you have $10 mil in Advanced Scouting. Will players just as often exceed projections as fall short of them, or will the projections typically be inflated?

imo you'll see much more inflated projections

Not only inflated but imbalanced. Like you'll see an 90 Overall hitter with a 10/95 vsL/vsR split. Could happen, but more likely it's a giveaway that what you're seeing is wrong. A very common thing is pitchers with totally unrealistic Dur/Sta, like 42/98.

If you are going to rely on adv scouting or really any scouting you need to use 20. If not then it will be less accurate.