Most people here on the forum replace the RFT with similar-in-price non-RFT for performance and comfort reasons, not price point. However, non-RFT are generally cheaper than RFT, though not by much. The most common choice here seems to be PSS for Max Performance Summer, and Continental DWS for A/S
Since there is no space for a spare, many people get the Continental Comfort Kit to plug the hole up on the spot.
http://www.tirerack.com/accessories/detail.jsp?ID=38
Or, if you like to pay a premium for the brand and brand alone, the BMW Mobility one:
http://www.amazon.com/BMW-Genuine-Fa.../dp/B000P5JO5M
Those are the "can of goop and air" equivalent of your Miata.