Depends what kind of wheel you want. Like, the excellent example you gave of rebuilding old rims and hubs-- in that case it can save a lot of money. Or, if you want something slightly unusual. For example, I wanted a Surly fixed/free hub built with a 700c Mavic MA3 rim. Although not exactly obscure, this is uncommon enough that it was probaby a little cheaper for me because I built it myself. If you want something very common, like a pair Ultegra hubs with Mavic Open Pro rims, then it may even cost MORE to build it yourself. At least in my case, however, the honest answer to your question is that I probably haven't saved enough (yet) to justify the cost of the tools. Then again, I find the process very satisfying, and I feel proud of the job when I'm finished.

Like others have mentioned above, the spoke tensionometer is not absolutely necessary, but having a properly-tensioned wheel is indeed worthwhile. That is, it can be done by sound or even just feel, but if you like to have some reassurance the Park tensionometer works well, and in my opinion it is worh the cost. That said, perhaps if I learned to built wheels without it my feeling of satisfaction would be even greater... alternatively, I may spend a lot of time retruing improperly tensioned wheels

Sincerely,

Henry