I think the Solstice will be in the low 20's while the Miata in the mid to uppser 20's price range.
At the same time if don't mind early cars yet sporty fun, look into the MGB GT or Midget convertibles. Ealry years like 69-73 had about 100 hp but the weight was low and it handles very crisp.
I got a 69 MGB GT (coupe) and am planning to get a convertible this summer (they go for 4-5k). Pretty fun to drive with a manual rack and pinion yet great on the mileage. Sean Gohain `03///Zed4 A thing of beauty is a joy forever, as its beauty increases it shall never pass into nothingness