You really don't want to work it out after the fact, or let your significant other work it out either.
My e34 M5 worked out at £1000/day - including EVERYTHING:
- track day - £200
- tyres - £200 (£600 set lasted 3 days)
- brake pads - £100 (£300/3 days)
- brake discs - £100 (£600/6 days)
- oil services (£60/day)
- insurance £100/day (but only at tracks/TDOs I've never used)
- petrol - £150/day
- accelerated wear on components - £200 (e.g. £2000 to replace adaptive suspension every year)
It did start getting cheaper once I bought smaller track wheels and found a source of relatively cheap track rubber (17 Pirelli P-Zero Corsa tyres from a Porsche Cup racer for £70) - and I could fit a spare set, and all my tools in the back of the car.
The Z4MC worked out about £800/day:
- track day - £200
- tyres - £150 (£600 set lasted 4 days, and have no space to carry spares)
- brake pads - £100 (£400/4 days)
- brake discs - £50 (£300/6 days)
- oil services (£60/day)
- insurance £100/day (but only at tracks/TDOs I've never used)
- petrol - £100/day
- accelerated wear on components - £100/day
Don't forget that in my case the brakes/tyres all did 25,000 road miles a year too - so some of the cost can be offset slightly. However, if you add 'upgrades' to the Z4MC costs, then it's probably similar to the M5
I tend to not do the 12 a year I was doing, and stick to 2-3 trips of 2-4 days long at the Nurburging which makes it more relaxed than our old 'there & back in 24 hours' escapades. I spend £500-ish on a 20-25 lap ticket (price depends on whether it's a peak or off-peak day, and some of my friends who go more often buy a annual ticket which works out much cheaper for them) and then there's not a lot of brake/tyre wear in comparison to a short(er) UK track. This then lets you space out your laps over the course of a few days, lets you drive around the area at your own pace when the track is closed, and gives you a bit of a holiday too. Although you then have to factor in travel/hotel costs.