There is also the upgrade cost to consider. If you already have a J model and you go 160 knots on 9.5 gal/hr and you miraculously manage to get a 200hp diesel to fit still doing 160 knots on 4.5 gal/hr here are your annual fuel differences:
100 hours per year:
100LL - 950 gallons at, call it $5.30 per gallon = $5035
Jet A - 450 gallons at, call it $4.95 per gallon = $2227.50
150 hours per year:
100LL - 1425 gallons at, call it $5.30 per gallon = $7552.50
Jet A - 675 gallons at, call it $4.95 per gallon = $3341.25
Seems like a big difference and the more you fly, the more it makes sense to upgrade to this hypothetical engine. However, I would guesstimate the engine swap would be in the $40-45 AMU range. The existing IO-360 is costing $50.35 per hour on 100LL versus $22.28 per hour on Jet A for the theoretical diesel on the prices I used (sourced from averaging numbers on 100LL.com). That would take about 1500 hours to justify the cost of the engine swap; 3/4 of the way to TBO. Meanwhile I think most of the diesels that did come to market had lower TBO than the proven gas engines. Even if you did the swap at overhaul, there is probably a $10-20 AMU difference in cost and near 1000 hours of flying efficiency gain to earn back your investment.
Now if you could get a 200hp turbo diesel for similar cost ... that is a different story, but also very unlikely. And don't forget the weight penalty; diesels weigh considerably more than gas engines for the same power.