Bought my astra 3.5 years ago for around £8K, with 20,000 miles on it. I've put about 55,000 miles on it and I'd probably get £4-4.5K for it now. So I'll let you work out whether, with depreciation, it's cost me more or less than your petrol omega. Bet it's more.
depends how you want me to present the figures 
you've done 55,000 miles and lost £4,500 in depreciation
8.2 pence per mile depreciation 
me zero pence per mile depreciation  

per 3.5 years
your 55,000 miles cost 12p  

 x 55000 = £6,600 in fuel 
my pro rata 10,500 miles in 3.5 years cost 24p x 10,500  = £ 2.940 in fuel
if i did 55.000 fuel would be £13,200  

  but I don't  

I do less miles in a car that suits my needs 
you do more miles in a car that suits your needs 
If I was doing 18 thousand odd miles PA, i'd LPG my Omega 
at 3000 mile PA ,it's not worth LPGing it 
Mrs Builder's astra H 1.6 petrol estate does 8,000 miles PA, @ 40 MPG also no depreciation ,14 pence per mile
Bro's astra 1,7 CDTi does 3,000 miles PA 55 MPG also no depreciation ,10 pence per mile 
"swings and roundabouts" "horses for courses "