Plug in voltmeter

I'm looking for some advice on a voltmeter (preferably digital) that
will plug into the "cigarette lighter" of my airplane. I found one on
the J. C. Whitney site, but $50 is too much for what I want to use it for.

I've got a JPI analyzer which reads voltages off the avionics bus, and
also a Davtron clock which does the same. Curiously, they differ, with
the JPI reading 0.3 to 0.5 volts lower! Furthermore, the voltage on the
avionics bus seems to be less than I'd expect, anywhere from 12.2 to
13.0 (instead of the 13.2 I'd expect). Further-furthermore, the voltage
drops during flight (I guess as the voltage regulator cuts off the
alternator as the battery is recharged after starting) to the point
where I get low-voltage warnings from the JPI.

No other symptoms of electrical problems. Been doing this for many
months (since I had the JPI installed) but still cranks fine.

Anybody know of a reasonably priced gadget that will let me monitor the
MAIN bus feeding the cigarette lighter?

Rich