Cahill, 22, who has been out on loan at Sheffield United and Burnley, could make his debut against Reading on Saturday but Megson believes the club have been forced to pay higher fees than they would have wished.
Megson said: "We have paid too much like we have done in every transfer.
"But that applies to every deal done in this window. The prices are really inflated.
"Having said that we are glad Gary is here. He is a good player now and will get better.
"It has taken a lot of negotiation and had it not gone through now we would have tried to sign him in the close season."