It's not the sills themselves, but the added work that if I were to keep the car I'd want doing as well (sills, cavity and under seal, dent removal, etc.). The front chassis rails aren't too bad, yet, but there is some surface rust appearing on them and it'd need doing as well.
It's the cost of everything mounting up that is rapidly making me fall out of love with the MX-5 really, which is why I'm considering changing it for something that'd essentially be more practical.