Gas Vs Diesel for economy.

Taking an average small cheap car:

1996 VW Jetta 

Diesel  Milage  rating 44 Mpg
Gasoline Milage rating 39 Mpg
.gov Source

on 6/2/08 the average cost of fuel in the US is: Source

Diesel   : 470.7  Cents / Gal
Gasoline : 397.6  cents / Gal

That makes the cost per mile for the car =  [(Cents/gal) / (miles/gal)]

Diesel      10.70 Cents per mile
Gasoline    10.19 Cents per mile     *** Gas is cheaper  (95%) *** list the cost of a private party 1996 jetta as:
Gas 	- $1500			     *** Gas is cheaper  ***
Diesel  - $2000

Seems like ebay has them for about 1200 to 2500 

The diesel could be a little better if you wanted to run biodiesel, 
   but a modification kit may be as little as a filter or as much as $3k