I went to the AH yesterday and spent maybe 80 grand upgrading my level 40 barbarian's various armor parts (I think I upgraded every single part spending between 1k and 7k per part). I was just looking for 50/50 str/vit as a base search and then scanning for good deals. Usually I could find something much better than 50/50 for at least one of the stats. For boots I think I had to ditch the str (high strength just doesn't show up on cheap boots). I actually bought one pair of legendary bracers entirely by accident.
http://us.battle.net...tem/slave-bonds
It only cost me about 5k and it had the str/vit I happened to be looking for, plus movement speed! When I was finished the only glaring deficiency was that I had zero bonus to gold/health pickup radius which is just intolerable. So I spent 20k to get rare pair of gloves with even better str/vit bonuses than the pair I had bought a few minutes before plus a 6 yard radius.
In theory, the amount of gold and items in the economy both grow continually, but it's currently hard to tell whether items or gold will grow faster. Depending on which, you will have the value of gold increasing or decreasing relative to items. I think items are increasing faster because a lot of gold is destroyed (15% for each auction and anything you spend in game) which means saving gold is worthwhile. However, I have no patience for that.
Using the AH effectively seems to be a skill in it's own right. Searching for the highest bang for your buck is different for each armor part. Boots don't like to spawn very high str and vit at level 40, so a search on that turns up really expensive stuff. If you search for the right stats on the right parts though, you can put together a great set of boosts for relatively little.
EDIT:
Another note. You shouldn't have to spend more than a few grand a part for solid items fitting your skill level. If you need items that are very powerful for your skill level, then you pay. So look for the best values, gain some levels and then do it again with new best values for the higher level.
EDIT2:
Also, I bought one item for 500 gold that was waaaaaaay overpowered for that price. I'm thinking the guy meant to put an extra zero on it. Since the AH hides the buyer and seller there was nothing to be done bu6t snap it up before someone else did. Can't even remember what it was... I'll check my logs later.