import re

pattern = r"(\"[^\"]* cost\":\s*)\"(\d{1,3}(?:\.\d{1,2})?)\""

s = ("\"apple cost\": \"2.78\" \n"
	"\"orange cost\": \"12.59\"\n"
	"\"melone cost\": \"42.12\"")

print(re.sub(pattern, r"\1\2", s))