Oh and you seem to have added £50 from thin air.
I was just sticking to the figures I quoted originally.
Though having checked, the price of a non BMW branded Bosch MAF is £208.
So there we go, less than a grand to replace all 4. Bargain! They are common enough failures that some of the OCD yanks keep spares in the boot.
The problem with posting on internet forums is that its difficult to seperate what somebody has posted before from what they post now when forming opinion (Happens to me, too, must be every other month somebody reminds me I once crashed). And you've spent years on here posting about how much debt you are in, how you were nearly bankrupt, etc etc yet now its time to run an E39 M5 on the cheap? I thought you were emigrating to Australia anyway?
The reason why £6k M5's exist is because people think 'How hard can it be?', buy one, realise how hard it can be, and then it gets moved on. They are yet another example of a car that really has no market other than neive people. I appreciate you are in a better position than many because of the fact labour is less of an issue (Though I say less of an issue not 'no issue' - unless you are an experience M5 specialist, in which case why are you even posting here about this?) but labour is only part of the story.
So there we have it - the answer to your question is that they are cheap because nobody wants them not because VED is going up (It already has - to £260), not because fuel is going up (Because it's been stable for 3 months), but because they are a massive liability at the bottom of the market and a medium sized liability at the top of the market.
You'd be well advised to see what PeterNem says - he ran an E39 M5 for 3 years so has practical experience of what they are like to run.
The E39 M5 is something i've spent a lot of time looking at - as most people know its my dream car - I almost bought one before and now I'm changing the 335i I'd be lying if I said I'd not strongly considered it again but each time it becomes very hard to justify the near certain massive expense.