Fuel in Bolivia is dirt cheap. It's subsidised so diesel costs about 34p/l. But the catch is you're only supposed to get it for that price if you're Bolivian. Fuel stations are supposed to fill out extra paperwork and charge you more if you're a foreigner - in some places they simply can't be arsed and the first place we went to (as we entered Potosi) told us they had no diesel even though there were two trucks refuelling. The second place further down the road charged us 83p/l so we got 30l to get us to Sucre.
When we left Sucre, one friendly fuel station told us we could get fuel for the cheap price if we had a fuel canister - if he filled the van directly he'd have to charge us the foreigners' price. But actually the easiest way to do it is to go to a rural petrol station. On the way in to the tiny village of Mizque, we were filled with diesel at 34p/l with no hesitation. Yay!