Disagree.
They are NOT leaches.
They are optimizing their business .. as You do.
And in general, ...
if the customers tend to find others to do their work ... and they keep in business .. your services are inefficient for that market.
But I do understand a part of what You say .. and I do agree with a part of it.
Many times, customers will imply ongoing future business, of much more scope, if You can only help them with this one bit.
And as You seemed to say, they do the 5 parts with You, and then go for the 10.000 parts with someone else.
I had huge success with this, on both sides, for many years.
By looking at first principals.
Look prospective current buyer in the eye. Must be owner/manager/decider.
"Listen, I *will* do this to You for a loss - for me".
*Now* - I will also ask that You sign a MOA/comfort letter/preferred supplier/commitment to buy XXX similar widgets from me, for 5 years or 10M pieces.
As You said You would, now, to get this cheap price.
And likewise when You go from 10.000 / set to 100.000 / set "I will" adjust the pricing to be in-line, and fair.
You are perfectly right that You lose money on the 5 piece jobs.
But You fail to see that 1/20 of those will become major good customers in the future, of great loyalty, and great profitability, IF and only IF YOU can manage it well in communication.
You might "lose" 500$ marginal costs x 19 = 9500$ per year.
But You would most likely gain 2-3M$ gross margin from the 1/20 who succeeds and sticks with You.
I think we just have a unique skill to attract low pay parts. .. Need 5 parts that you can make on a bridgeport..... We don't even quote em.
I know some probably think that is the wrong route to take but we will literally lose money doing them and in our experience, our customers just use us until we price something with good profit, and they move to someone else.
Leaches really.
). 
