Why sell your mazda3? The fuel that you'll save from buying a jazz will just offset the depreciation that you absorb when you bought the mazda3 & for the new jazz.

Here's my computation considering conservative estimates:

Your Option:
P800K Mazda3 (new) 30% depn (now) = P240K
P690K Jazz CVT (new) 50% depn (a few years from now) = P345K
P585K in total depreciation expense

less P20K per year savings on fuel =
assumption per year: 15,000 km, P40/L, 8km/L MZ3, 11km/L Jazz

Retain Option:
P800K Mazda3 (new) - less 60% depn (after a few years) =
P480K in total depreciation expense