John,

Depends on a lot of factors. Let's look at the Thielert Centurion 1.7
diesel, not yet available in the US (due to stalling of the FAA,
AFAIK), but certified in Europe. AFAIK, the engine is 20,000 $,
including all accessories and a guarantee for 2400 hours (TBR) or 12
years. If something breaks before, you'll get the new engine prorated.

The conversion kit for the 172 is 40,000 $. The Centurion will use 4
gallons or so of JetA1.

If you calculate the difference between the amount of JetA1 used by the
Centurion and the Avgas used by an O-320, factor in the cost of the
engine per hour (guaranteed with the Centurion, full of surprises with
the Lyc), you'll find that the point at which you'll get the extra
20,000 for the conversion back depends very much on local fuel cost and
similar factors - but the point will come.

--
Thomas Borchert (EDDH)