I actually did put them on a scale. The stock HF wheels are 13" X (4.5?)"...anyway, they weight in at just a hair over 13LBS., if memory serves. (I will weigh them again both with and without the little "H" caps).
As for the Miata BBS rims, they're definitely lighter, by 3LBS or so...and larger too. The Miata rims are 14" X 6"...and the tires I got for them were Nokian's that came off of a Prius
The tires are 175/65/14's that are even lighter than the tires that were on the HF steel wheels. I did my homework on this swap! The Nokians can also handle 50psi, no problem -(not that I wasn't already doing that with the Aquatred-3's that were on the HF wheels...I was!).