The expense of rear tanks is a debatable point. Paying 1000 euro, for example, for 15 litres of rear tank seems expensive. A 40 litre front tank may cost about the same. 40 is a much bigger number than 15!
However, taking an Africa Twin as an example bike, fitting rear tanks would bring the total fuel capacity of the bike to 40 litres, which is exactly the same as removing the original front tank and fitting a 40 litre front tank. So for roughly the same amount of money you end up with the same total fuel capacity. Not an obvious conclusion.
Whichever way you look at it though, oversize or additional fuel tanks are always going to be more expensive than bungeeing a couple of jerrycans to the bike. The flip side is security, jerrycans can be easily stolen from a bike.