Hey there
I can attest that posties are indeed the cheapest form of travel, the costs are almost halved again if you ride 2-up like us.
We set out from Perth some 3.5months ago on our standard postie. It has an extra 7L tank from a chinese MotoXer so we have about 12L in total. On our one month journey from Perth to Darwin it cost us a total of Au$240 in petrol... Au$120 each!!!! We were constantly shocked at what other peoples costs were along the way.
Petrol was a lot more expensive in East Timor the 3weeks we were there, we spent a bit on petrol but considering the miles we did and the condition of the roads(read goat tracks) we came out in front of the huge slow trucks everyone piles into over there.
Now we are in Indonesia, been over a month island hopping. Because the postie is small like all the bikes here it can be loaded on small boats (and canoes) for often no extre charge. On land we have worked out that we are travelling a LOT cheaper than a local could by Bemo/Bus. A little cheaper than a local could on a motorbike because we are 2-up.

The bike is running fine but we are packing VERY light.
We are splurging a little here on accom cos we camped all the way through Australia. Its still cheap though, each we are paying about Au$5 a night. We save again by buying veggies at markets and cooking dinner in our room, we like to eat local food for lunch though.
We are about to head into Java then Sumatra... Our visa is running out and have to soon cross to Malaysia.

It can be done and the bike will still be there begging you to take it out on a long journey again. Be careful of the clutch though, nice crisp gear changes and oil changes every 1000kms will make it last longer.
We got a new one fitted in Broome.

Good luck.
Nathan and Akiko.

__________________
Round the world on a Postie bike, 2-up
Bilingual Blog (japanese and English)
http://faster-than-walking.com