I generally value them at €0,01, but that of course is only true when flying Premium Classes. For personal flights, I usually fly sale economy fares (so a TATL R/T would be approximately €600). If the same flight is 50k Miles + €370 surcharges, it makes them worth € 0,0046 per mile....

Edit: Actually even less, because flying for 50k, I would also miss out on 20k miles earned, so they're only worth €0,003 per mile....

I now booked a 100k Ticket to Australia and regretted it when I found a sale for €700,- oh well...