I encountered this myself and have a few conclusions:

1) Be open with them and tell them that without a cap table you have to assume the options are worth $0. Use that to push for more salary.

2) The reason for not disclosing that information is often that the company is close to another fundraising event (an IPO for example) and that is very confidential information. It's definitely not a bad thing.

3) A safe assumption (to use yourself, not in negotiations) is that the company will exit at around $10/share. There might be a reverse split or something so take this assumption with a grain of salt.

My experience was not that the company was being malicious. I joined and it worked out fairly well.