250cc vs 650cc

Roadtax RM50 vs RM250
Normal service same around 2l plus oil filter
Insurance depends on value insured if new r25 vs 2nd z650 will be about the same
chain both uses 520 but z650 sprocket set is slightly more expensive
Tire z650 a few tens of ringgit more expensive due to bigger size

What will get you is if there is something major. Major service like VC, top overhaul or accidents. Parts cost magnitudes more.
But bigger bike has its advantages. If you like to go fast. Probably you will save more fuel on a 650 than r25. Lower rpm and all that.