You might want to see how they play before calling them great players. The thing with Cahill is, like Young, you know for 98% certain he's not leaving for La Liga or any other top league in the future, he's also got proven EPL experience unlike both of the other two.
Mertesacker might gel great in our defence, or might be terrible, he's notoriously slow and that might make him near useless in our league, he might be brilliant, we won't really know till he plays for us frankly. With Cahill the higher price also comes with a near certainty he will continue to play at the same level he has in the league already.
Hearing that he'll also be on 80k a week I'd imagine he will cost at least 1mil a year more than Cahill would, so at the end of four years I wouldn't imagine the price would be much different.
The last problem being, we we just sign a CB, LB and striker, we haven't scratched the surface of what we need, of course, being halfway there is a vast improvement on our current situation. Central midfield is our priority along with left back.