1) Before you purchase an item follow one or two auctions of items with similar stats to gauge the item's comparative worth. MOST IMPORTANT RULE
2) Always realize there will be a "sweet spot" for the low end price range and outside this "sweet spot" prices will rise astronomically. As an example, my latest set of Inna's pants cost me about 6.5 million (173 dex/69 int). The following day, a set with 183 dex/80int went for 40 million plus on bid. Try to buy at the top end of the "sweet spot." In addition, note that the "sweet spot" is not static. Check back frequently to see if you can further improve your Monk at a reasonable price.
3) Try not to purchase dex/vit/AR/cc/cd on the same item - it will cost you. Much cheaper is to buy an item with one, two or three of these stats but load up on them. If you buy an item with no vit for example, make sure you compensate for that lack of vit by loading up vit on another item and maybe leaving out AR, cc or cd next time. Once again using Inna's pants as an example; in the same time frame as I was looking to make my eventual 6.5 million purchase (173 dex/69 int), an item with 186 dex/30 vit went for upwards of 160 million. So 150 million plus for 13 dex/30 vit.
4) Buyouts can save you lots of gold, especially if you have followed step one and know the comparative worth of an item.
5) OWE is your friend. This step is also very important and can save you lots of gold. If you are poor, do not listen to people telling you to go OWE free.
6) Crafting is risky but usually rewarding, especially if don't insist on your crafted item having to be BIS.
7) Being outbid on an item is not necessarily a bad thing (see rule 1).
8) If you have the time (and the inclination), cheap purchases can often be made at off-hours. If most of the competition for the item is asleep, you may be able to pick up a bargain.
9) Do not put all your eggs in one basket. If you have decided to purchase an item, it is a good idea to put in 2 or 3 bids for similar items if they are at the start of their auction cycle and/or are currently available for less than 5K. This will give you multiple shots at your attempted purchase, will put the items in your auction list and will also give you more info about the item's comparative worth (rule 1 again).
10) Nothing is cheaper than a self-found item. So fellow Monks keep on farming and good luck with your next purchase.