Replaced new faucets in bathroom to find No leaks at coupling nuts but
wet at (rubber)
black base plate and some drips at plastic mounting nuts under sink,
any installation
tip or should just replace for more expensive ones. my purchase
about 39.95 each.
thanks for any help.

1/8" of plumbers putty onto the sink from the holes to the edge of the
rubber plate before you put down the rubber plate.

you may need to replace the old lower compression nut's bushing and the
vertical supply tubes if tightening nut does not stop drip. take along
old parts to hardware store.
you may need to check top and tighten its parts if it is seeping out a
stem.