lee i have a 2003 astravan 1.6 duelfual,
it does about 31mpg average over 1 tank of gas about 360-380 miles which costs about 22 quid, (54 litres)
i used to have a diesel astravan before and it would cost me 40 quid of diesel to do the same mileage, when diesel hit its peak last year lpg only went up to 49p per litre (diesel was 130p) so filling my van went to about 25 quid.
lpg is dirt cheap i would never drive a diesel again, my van has a factory fitted lpg kit by vauxhall and has been ultra reliable never needed anything in the last year and a half that i have owned it, and i have put 30k more on the clocks
i paid 3400 for it but you can pick them up for about 2-2500 now