It's certainly not ATPL knowledge. Can't think why you'd want to know this. (Not that this would stop it being examined for ATPL, but that's not the point!)
I have absolutely no idea about the weight. The best thing might be to contact an avionics supplier. Or possibly your local PFA group - someone must have a compass waiting to be installed in a home-built that they can weigh for you. Of course this won't give you the weight of your particular compass in your particular PA28, but.....
The lever arm you can find out from the POH. It is measured in inches forward or aft of the datum. Normally you just treat these as meaningless numbers - but the POH will actually tell you where the datum is, then you can measure (with a tape measure) the distance from there to the compass.
I'd be more interested in hearing why your instructor wants you to find this out than I am in hearing the answer the to the actual question!!!
FFF
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