Firemaking is the cheapest. Costed me about 5-6 mil to purchase about 90k maple logs.
Cooking is prone to burning, that's how I look at it.
When I was getting my firemaking cape, I clocked the XP grind around 130k/ph. Which was around 1100 logs burned an hour, give or take a few 10's. Took me about 2 weeks of nonstop burning to get to 99.