For the sake of argument, and I'm sure there will be some, lets assume you
have a 1989 Westy Syncro.  Assume the price per gallon differential between
Diesel and Gasoline is 10% (3.00/gal vs 3.30/gal). Assume the fuel economy
increase will be on the order of 30%.  Assume you will drive the converted
van 100KMi.

If you drive 20KMi per year that's 5 years to amortize the cost of the
conversion.  10 years if you drive 10KMi per year.  If the conversion cost
is $15,000 then you have to save $1500 on fuel for a 10,000 mile distance.
Thats' roughly $15 per 100 miles travelled.  If your gasoline consumption is
20MPG now and 100 miles costs 5 gallons or $16.50 and your new diesel
consumption is ~30MPG and that's 3.3 gallons per 100 miles or ~$10 it's
pretty clear that amortizing the transplant cost will take more than 100Kmi
to break even, more like 200K miles or more.

So a TDI conversion for fuel expense reasons is at best a very questionable
venture.  However if you're going to have to do a new engine anyway to the
tune of say $5000 then things become somewhat more reasonable.  You would
only have to save $1000 per 10,000 miles in fuel or $10 per 100 miles
traveled.  Still not home free but if it smokes your cigar then...

So to break even you need to get that transplant cost into the realm of $6
per 100 miles pointing to a $11000 transplant cost (taking into the mix the
fact you'd have to spend $5000 anyway to fix your present motor)

Lots of other factors to consider but putting the maintenance and such aside
a turnkey transplant gross cost of $10000 is a pretty good breakeven point.