Will it cost you more to deconvert that cab than it will to pick up a complete Tempest elsewhere? Yes. But that alone is not necessarily a reason not to do it.
It depends on what your goals are, IMO.
If your goal is simply "to own a Tempest", then yes, there are cheaper and easier ways to get one. (Though personally, I think the days of $500 Tempests are rapidly coming to an close, as more people become aware of how much the parts are worth, and how sought after they are with all of the Quantums and MH's people are building and reproing.)
But if you're looking to actually rescue and restore a game by hand, from the ground up, then you will get far more satisfaction from deconverting that cab, than you will out of buying a different one. The parts can all be obtained, with money, persistence, and a little patience.
If you are the kind of person who genuinely enjoys building, learning, repairing and restoring things, then you will be very happy with the final product if you choose to deconvert. If you are not, then don't do it, as you will come to hate the experience.
Will it cost you more in the end than buying a complete cab? Yes.
Will it give you as much personal satisfaction to buy a complete cab outright, versus looking at a game that you love, that you know you put your heart and soul into bringing back to life, and have made uniquely yours in the process? Maybe.
If the idea of doing the latter appeals to you, then the EXPERIENCE (not to mention the knowledge you will gain by assembling a vector game from scratch) will be worth whatever it costs you to do it financially.