General Rule of Thumb I use for myself:
1. If it's to high, DONT BUY! NO MATTER WHAT KIETE, YOU KNOW ITS NOT WORTH IT, DONT BE A DESPERATE IDIOT.
2. Wait a day after tier to look for item of interest, so that rich people can buy at absurd high price first and get them satisfied.
3. Ask friends who are buying AP if they will get your item of interest for a good price (makes me feel better if I know the money is going to someone I know and in a way you're helping them)
4. Resist temptation to buy AP, unless its impossible for you to purchase item with gold Q-Q
5. If no option left but to buy AP, plan out what your going to buy, and keep an eye on market of what people are "Wanting to buy" vs not. Get the item you want, sell the rest for profit or use for gearing.