My short list at the moment is:
E92 M3 $65-67k
E89 Z4 ~$64k
F355 GTB $60-70k
Yes, you can pick up a Ferrari F355 for the same price as a new Z4 (just check
www.autotrader.com). Now obviously maintenance and insurance is going to be more, but then you won't have as much depreciation.
I guess a 2 seater roadster should command a small premium over the 4 seater E93 335i, so maybe the issue isn't so much the high price of the Z4, but the relative bargain that the M3 is.