Cahill finally signed for the Blues yesterday after weeks of talks and speculation.
The defender would have been out of contract this summer, meaning that the Trotters had to cut their asking price for the 26-year-old from £18m.
"Chelsea are getting a bargain, no doubt about it," Coyle said.
"If it was a level playing field, he'd be worth a lot more than we received."
Bolton face Macclesfield in the FA Cup tonight.