How much is one of these worth?

The only use I would see is rainwater harvesting for a tillage man for spraying. What's it worth as scrap value by weight? That plus 50% at most would be my guess.

I always wondered why a contractor wouldn't buy one, and fill up when the price is low like last year

Cost would be the biggest thing would it? And then theft like Moo said... There is only so much stuff you can carry, without maybe an overdraft taking the good out of it with interest rates...

It's very simple really, cash flow. To fill one of them when diesel was back at say 65 cent would still cost over €20,000. It's a well set up contractor that can take that from his cash flow in the summer and leave it sitting there for 6-12 months until the prices rise. I'm sure there are some that would be able to do it but for the vast majority it wouldn't be possible.

There is also the theft problem as well, and as Mooo said, them lads won't mind drilling it. There's a small BnM bog up the road from here with a 10,000 litre steel diesel tank. A few years back it was drilled one night, a 1,500 or 2,000 litre bowser was filled from it and then they drove off leaving the rest to pour out on the ground. The tank was well empty the next morning and a flood of diesel across the ground. Them lads just don't care.

Depends on how close you are to south Armagh, the closer the better!