RE: regular voltmeter and onboard?
It depends if you want a hand made kludge at the field you have to fumble wth or a compact sealed unit that is simple to use and bullet proof.
Yes, there are always ways of doing it cheap. To me $40 for an invaluable tool that I only need to buy once IS cheap.
One can only squeeze a penny so tight - sooner or later Abe Lincoln's gonna SH*T in your pocket.
DP