It also would not have been chivalrous to kill an opponent who yielded, so the only ones dying are those who were killed in short order on the battle field, or those who refused to yield in hopeless circumstance.
Ransoms were really, really large in that period. Five years after this battle, the English captured the French king at Poitiers. His ransom was 4 million gold coins plus, I think, various concessions. The French ended up having to pay a lot of ransoms through the whole war, and as a result, the nobility and crown were so poor they couldn't prevent banditry and famines.
Barbara Tuchman's A Distant Mirror: The Calamitous 14th Century is a very engaging account of this period.
You can't bribe a fanatic.
According to the visitors' guide to the Chateau de Josselin "As evening fell that day, the victor, Captain de Beaumanoir, returned the prisoners to Josselin and had them executed."