As there is civil unrest at the moment in Burundi, it is worth considering what peace is worth, just in terms of lost income per person.
The graph below shows income per person in the DRC from 1990 to 2013. There was full war in the country from 1996 to 2003 (with some conflict persisting afterwards). Income only starts recovering after 2002, when the war ends, and only reaches the pre-war level in 2003, or 18 years later. So 18 years of development seems to be lost. Income was falling before 1996, but the country before 1996 was under an entirely different regime with a long history of reducing national income, so can perhaps be considered to be fairly distinct economically from the country after 1996.
Source: World Development Indicators
If the DRC graph isn't convincing, the Burundi graph below shows that since the civil war in 1993 (which became hot and cold by turns over the next two decades), income per person hasn't recovered.
The clearest example of the effect of a war comes from Rwanda in 1994, which was brief and followed by unbroken peace from 1995 to 2015. The graph is shown below. Income per person almost halved in 1994, and only recovered to its pre-war level 12 years later despite successful economic management.
The decades of lost development due to recent Great Lakes conflicts, applied to the millions of people in the region, resulted in hundreds of billions of US dollars in lost income, I think.