If these are the famous hollow-spoke Miata wheels, probably these:
Typically 100x4 lug spacing, full hollow spoke 14" are about 10 lbs, semi-hollow in the 11 lb range, solid spoke in the 12+ lb range.
Julian's Home Page - The secrets of the '94-'97 MX5 OEM Wheels: Hollow Spoke and Solid Spoke - November 13, 2002
ROTA also sells really light cheap wheels. They have a set of 15x6.5 wheels that are 10-12 lbs, called AutoX... but I doubt you can find them secondhand as cheap as Miata wheels.
If your PCD is the same as the Miata's, I've always found this page helpful:
http://www.miata.net/faq/wheel_weights.html