What's more affordable on the used Vette market, a late C3 or early C4?  

Right now from everything I've seen, they appear to be about the same, outside of the '82 models.  But right now everything from a '75 to a '91 seem to be going for around $10k on average, with certain exceptions of course.  But it's definitely a buyer's market for those models, and maintenance costs should be fairly reasonable yet even if they require an overhaul in the drivetrain.