"War is always more costly than peace," he said in an interview with the BBC.
Mr Santos and Farc leader Timoleon Jimenez, known as Timochenko, will sign a historic peace deal later on Monday.
But it will take a long time for Colombian society to recover from more than five decades of conflict, he said.
"We could have grown between 2% and 3% more per year for the past 23 years," said Mr Santos.
The conflict has also had a profound impact on Colombian society, Mr Santos told the BBC's Lyse Doucet.
