Usually the common stocks goes at a discount of around close to 15-20percent when compared to preferred for good deals. If the company is not doing well, the discount would be higher proportionally.
Founding and working for multiple startups, including some that were fairly successful and therefore attracted multiple rounds of funding. I can't claim to have as much data as a VC would have, but I certainly have seen wider spreads personally, so I can attest that they exist.
I wish there were a better clearing house for this sort of collective experience. I guess HN serves that purpose, to an extent, but it's hardly organized.