It is likely to depend on whether you want Status or Award miles.
Award ones, outside an offer, have traditionally been somewhere around 1.5 to 2 Eurocents per mile, likely plus a flat transaction fee (maybe 20 Euros, based on the cost of booking a flight through the call centre). However, this was in the days of much better earning for flying with almost all Y fares earning at 100%, so this may well now be anything up to double that or more to encourage people to fly rather than just buy-up.
Status miles, frankly, are anyone's guess - it's difficult to imagine someone who needed 2,000 miles to re-qualify (which is the buying cap we're expecting) being able to buy that number for as little as 80 Euros plus fee (which would be the cost if Award and Status were priced the same). I would expect it to be something like 5 to 10 times that amount given the benefits.
Last edited by NWIFlyer; Oct 1, 2015 at 8:35 am