The problem with simple maths is that, well it's simple. But here's some maths on petrol v diesel. All these figures are subject to variables of course and people will always dispute them, but they're intended as a comparison.
Let's take 10k PA miles as a bench mark. I don't do 10k in my 80 for sure.
In a diesel doing 19 mpg (I'd kill for that. I've had as little as 15 mpg but it hovers around 18 mpg. With current pump prices taken as an average from the RAC, this would cost you £2746. In a petrol doing 11 mpg (bear with me) the same mileage would cost £4498 at the pump. That's a difference of £1752.
Now if we could squeeze 13 mpg out of the petrol and we know more is possible, then that difference comes down to just over £1000 on 10k miles. If you only get 17 from your diesel and 16 from your petrol, well!!!!!! I've driven the petrol version and it's sublime. But in order to get decent mpg you do have to stay off the loud pedal which isn't easy, trust me.
I'm not trying to convince anyone that running a petrol is cheaper or even cheap. But it's not as horrific as you might think. Given that you could find one cheaper than a diesel, lower miles, fewer owners, better condition etc then it has to be a possibility. I know people who have bought cheap 80's (£3500 ish) and spent more than that on parts and labour to get them into half reasonable condition.
The UK has some of the highest fuel prices around. Most of mainland Europe is pretty similar, but once you get out of the really civilised but - where you will want to be going on your travels, it gets pretty cheap. In Russia we were paying just over 40p L for diesel.