In 99% of the cases the plane is going there regardless of what cargo is onboard. Revenue Mgt at the airline I work for does not even take into account cargo revenue when determining route feasibility. It''s just seen as icing on the cake.
I think it's a big problem for most of the airlines - they don't put enough focus on cargo! I don't know many airlines that would've change pax a/c type because of cargo demand!
Of course you also have costs to move and handle the cargo so it is not just "icing on the top". It needs proper planning to achieve the "icing". Normally airlines cargo department has their own revenue management to calculate the revenue of cargo separate from the pax reveune.
When it comes to rates of cargo it varies a lot depending the kind of goods you're transporting - if you're handling special products that require some sort of extra or special handling the rate is also higher because the airline probably also has more costs for the extra/special handling in form of extra processes/equipments/facilities/staff etc.
Rates also varies a lot between origin/destination you might have EUR4
/kg from Asia to Europe but from Europe to Asia rate for same piece of cargo could be EUR2
/Kg on the same a/c (this is just an example not realistic figures!). It's all simply because of the market differences and the "demand".
"High yield" or "premium" cargo is of course what the airlines want the most because the profit is better than from the "general" cargo. This could be Express, Dangerous Goods, Valuables, Vulnerables, Perishables, Live Animals etc. where the rates are higher than for "general cargo" - and there is lot of specials flying around!
It's bit like that every airline would love to fly only First or Business class passengers if there would be demand for it!
In general, a well organised cargo department is a good profit maker for airlines, problem is that many airlines (specially in US) handle cargo with their "left little finger" and don't put attention to it that it needs!
I like driving backwards in the fog cause it doesn't remind me of anything - Chris Cornell