how much does a weather diversion cost?
 

If aircraft flying into London Stansted (STN) have to divert to Luton (LTN), due to fog or low visibility at Stansted, how much does it cost to ferry the empty aircraft back to Stansted? (LTN and STN are about 23nm from each other in a straight line)

There is a free route economics calculator at Data & Analysis Products for Aviation | Aviation Analytics which you can use to work out sector costs between any two points within Europe.

It's more complex than simply the cost of repositioning the aircraft. There may well be a knock-on effect causing further delays and cancellations, compensation, passenger reaccs, and so on.

I watched a 737 leave Southampton for Bournemouth after a diversion a couple of years ago, it went up like the space shuttle!

it can be in the region of ten's of 1000's of £

depending on a/c type the handling fees, if crew go out of hours, Hotac meals or transport for pax if flight is terminated there

it goes on and on plus the hassle to the next flight(s)

One thing is sure: it costs less than a get-there-itis induced accident. Both in the long and in the short run.

Thank you for the replies so far.
I had in mind a specific scenario that could easily apply to either of our favourite low cost airlines.
i.e. on arrival at Stansted runway visibility is below allowed limits so a ROUTINE diversion is made to nearby Luton where the vis is within limits.
It is mid-evening so aircraft has not finished for the day.
Within, say, an half an hour weather improves - so is aircraft flown back to Stansted to continue scheduled next flight, although late, if that is what would happen. Crew still in hours, so far.
So extra distance flown,extra landing, extra take-off, more taxy, more landing/take off fees and ATC fees, more tyre wear.
How much fuel would this require? What would the extra maintenance cost be for the extra time aircraft was flown? Is the extra cost significant? How would this compare with the extra fuel usage for a 30 minute cross country taxy at, for example, Amsterdam or Madrid. Is it a big deal?

I have in mind like PPL training where each touch and go cost twenty quid and took 10 minutes off the 50 hour check.