Ape Atoll is certainly the best place to go about chinning, assuming you can afford the supplies. As for the cost, it can vary depending on the equipment used. Personally, I find that normal chinchompas, give 2/3 of the exp red chins do, yet cost only half of what a red chin is. So for me, normal chinchompas are a bigger bang for their buck (literally). It all depends on your own style. I use approximately 18 prayer potions along with 2500 normal chinchompas and get close to 450k xp in one trip, which averages close to 2 gp/xp.
Hopefully that info helps you make a decision.