I don't know what your friend is charging you, but I don't think MFJ diplexers are less expensive than competitors. If it's free then it's worth what you paid for it.
From Ham Radio Outlet:
Diamond MX72H 1.6-30 + 49-150 MHz low pass, 400-460 MHz high pass, 60 dB isolation, mix SO-239, high and low PL-259 on cables
$49.95
Comet CF-416A 1.3-170 MHz low pass, 350-540 MHz high pass, 60 dB isolation, mix SO-239, high and low PL-259 on cables
$54.95
MFJ-916BC 1.3-35 + 50-225 MHz low pass, 350-540 MHz high pass, 60 dB isolation, mix SO-239, high and low PL-259 on cables
$79.95