Allowances as income??
As alluded to above, your consideration of allowances as part of your income planning is nothing short of complete folly. This is the mentality that has permeated throughout the cabin crew world, and it's no secret what sort of rubbish money they are on.
Outport allowances are not contractual, they are the same for us & the girls, and are re-assessed regularly by "the company" based on prices at the crew hotel coffee shop / restaurant. So, if the price of noodles goes down, so does the allowance. If the price of steak goes up, the allowance doesn't necessarily go up. Like anything else by or from "the company", it is totally arbitrary. The allowance only goes upwards if the cabin crew start to make noises about the expensive prices of asian food - the company doesn't give a hoot about our opinions in that regard. For example, the company now give cabin crew on the DXB/JED shuttle an extra night of allowances (we don't get it), because the sickness rate for the girls was out of control.
Additionally, your estimate of the amount of cash being based on 3 - 4 trips might sound right for an SO at the moment, but it is not guaranteed. Next time there's a reduction in flying, you will do less overnights. I'm not just talking about GFC, SARS etc, but there are fleet changes happening soon at CX - If you are on a new fleet such as the 747-8 or the 350, for the first half year or so of the introduction, you will not be on a full roster (ie less overnights) as the fleet is bedded in. Conversely, as a fleet disappears, someone has to stay until the end, which means less overnights.
FOs & Captains who fly regional type fleets get more overnights, but less cash per trip. Plus, the cash you get in local currencies is not worth much in a conversion to HK$ (JNB, TPE, BKK are just a few fine examples). When you are on leave, those allowances aren't there, but maybe that might be an advantage of having only 3 weeks of leave per year when the rest of us get 6.
Besides, this discussion is of nil consequence, the purpose of the outport allowance is for your/our/my expenditure for food etc whilst we are at the out port AT THE COMPANY'S BEHEST. If you don't spend it all sometimes, then so be it, but there are a few cities where the allowance can disappear within hours of arrival (NY is an expensive example).
If you want to be like the girls, who go all around the world, never go out, and only eat cup noodles in their room because they are trying to save their allowances, then go for it.