Since Cahill left for a reported ?7million, Coyle has brought in another centre-half in United States international Tim Ream, 24, from the New York Red Bulls, and 20-year-old England Under-21s striker Marvin Sordell from Watford.
And given their respective ages, Coyle believes their acquisition also represents good business, and said: "I think we all accepted that Gary was going to go and once it was decided on the football side that it would happen, what we had to do on the business side was look to try to maximise the situation that we were in."
He added: "In doing that, we have then reinvested that money. We have not gone and spent the money on 32 or 33-year-olds where there is very little or no resale value.
"We have tried to invest it in young players who will become assets for the football club and whose value will appreciate.
"That is opposed to a quick fix, somebody who will do you a turn for a year and then all of a sudden you are back to square one. We are not in a situation where we feel we need to be doing that.
"I always look to try to put in place a plan where you invest in youth and bring them on year by year."
Sordell's move was finally confirmed in the early hours of Wednesday morning, and the player revealed he knew nothing about it until the last minute, saying: "I heard about it on Tuesday (deadline day). I was originally set to go to Cardiff and I got a call later on asking if I wanted to go to Bolton.
"Obviously I said yes, it was a no-brainer really. Straight away I said 'let's get it done'.
"I didn't expect it at all. I was ready to stay at Watford the rest of the season. Now I just want to do as well as I can here."