If we are to sign Gundogan then the fee we spend in relation to his contract length left at Dortmund is irrelevant IMO.
Gundogan is rated as one of the best CMs in Europe. He's 25, a current German international and has a skill set that our midfield has been severely lacking. I'd even wager that if he was to run his contract down then no club in the world would turn him down as he would improve any side.
In this day and age £30 million for one of the most complete midfielders in Europe, who has at least another 6/7 years at the top level is brilliant business.
If you were looking at other top midfielders such as Pogba, Xhaka, Busquets and Kroos etc then the price would either be very similar or a lot more expensive.