As Bango said you should not loose more than 20-30,000 of what you paid in total (lock+tire+reg+etc included). So you can make an estimate from that, Alto is a HOT CAKE of the market!
Yes i also own an Alto CNG of July 2010, and love it, had some problem with the hard steering so I installed EPS (power steering) in it and now everything is fine.
I have learned that mileage mostly depends on how you maintain the car, My mileage is,
CNG w A/C = 110-130kms... 15km/kg (aprox) (mostly 120km is the average but like I said it changes after a few(2-3) months and means it wants service, get 130 when I drive mostly on signal free roads)
CNG w.o A/C = 130-150kms 17.5km/kg (aprox)
CNG w A/C Highway = 150-170kms 20km/kg (aprox)
CNG w.o A/C Highway = 170-200kms 22km/kg (aprox)
Though I feel it should give more mileage... it can not give more mileage than this... unless I move to a cooler city like Lahore/Islamabad/Murree/Quetta...
CNG tank gets filled @ RS 460-520 depending on gas pressure (8-9kg CNG from 220bars to 240+bars)
Alto is not very good with petrol thats what I think 12-14km/ltr it the average I think.
If you are thinking of buying Vitz/Swift than I would say go for it and sell it if your going to buy Coure/Cultus then I will say just spend a little more and get a CNG kit installed into your car and have fun...
Or if you can pay a little premium sell the car and by a factory fitted CNG Alto, I do not know but I have a thing for factory fitted stuff do not like to replace/add stuff until absolutely needed.
Edit: Yes new petrol is going for 656K + 5K for metallic = 661K for petrol Alto.