Not very scientific i know, but:
In invasion I just got a 1 hit kill after 43 hits (2,500 sp spent) and got 69 bil Exp
Then I got 1 hit killed after only a single hit (60 sp spent) and got 18 bil Exp
Now, 43 hits (time, finger wear, and sp) only got me 4 x the Exp
It seems that the ratio of exp to sp goes down significantly. I know there are other factors and you have to factor in the cost per E, but is this fair?