Based on your experience, what are the odds of a draft prospect signing if he is listed as "Undecided"?

My experience is limited - I've only participated in 7 drafts - but in my experience he will probably sign. If you draft him, he'll initially ask for an amount that is higher than the slot recommendation. Once you make him an offer, he'll probably take a few days or even a couple weeks to get back to you. But once he does, he'll either agree to your offer and sign, or he'll ask you to offer more money. If you meet he higher demand, he'll sign.

I've only had a couple players completely refuse to sign, and I think those guys were listed as "probably won't sign."

Limited experience here too... but I've already seen a wide range of outcomes, not sure what the probabilities programmed into the sim might be.

Undecided means what arcticlegend said, note that yes you will have to wait on the player to get back to you - in my experience at least a week real time. I would say he'll probably sign, but I've also seen a "Probably won't sign" accept on the first offer, which in my understanding just wasn't supposed to happen.

Also, an undecided player might ask for a huge pile of cash. Happened last season, the #1 pick went for slot money at 4 million, the #2 pick ended up costing 9 million.