I've gambled a few hundred million in gold now (I would guess 700-900M) with my level 95 Trav runner. I'm not saying I know it all - because I definitely don't - but I'm not completely ignorant on the subject either. A few tidbits:
- I already have a very solid equipment pool, so I'm looking for near-perfect dual-mod magic items or 3-4 high-mod rares.
- I've focused almost exclusively on rings, amulets, circlets, and coronets as far as getting good items. Like Gripphon said, rings most consistently produce a "desirable" item, but as far as profitability & trade-fodder, I don't think they have any higher ceiling than any other base-type that I gamble.
- I also gamble belts in the hopes that they upgrade to Mesh Belt/Mithril Coil to be used in blood-crafting recipes (since they have 4 rows of potions). This is a great way to find crafting bases since you're looking for magic-quality items and 90% of your gambles will be magic quality (from my experiences I'd guess the upgrade-to-exceptional chance is 25-40% and upgrade-to-elite is ~5% for my level 95 character). But as far as gambling an actual "usable" item, my expectations are much, much lower here. I'd lump gloves & boots in with this last statement too (but I don't care about their bases for crafting, so I just shop them from vendors instead).
- I was never much of an assassin guy, so I've been skipping claws so far (but I might add them to my list now with some of the advice here; mostly because my rare claw collection is pretty sub-par).
- the odds of getting a "Jewelers Armor/Shield/Weapon of the whatever", or a particular unique item, are super-duper low... I think to get a Jewelers Monarch of Deflecting you're looking at tens of billions of gold spent on kite shields (and Tyrael's is well into the hundreds & hundreds of billions). Imho, trying to gamble these items isn't remotely worth the effort, time or resources you'll spend. Your odds for results-vs.-time-invested for many of the magic items are probably better running the vendors and shopping (or simply playing the game

).
As a rule of thumb, I expect to gamble:
- one "good" item every 10-15 million gambled (with Edge bow & Gheed's charm). By good, I mean something like a Scintillating Ring of the Apprentice with a high resistance roll or a Volcanic Amulet of Life Everlasting... i.e. something that's not always as useful as semi-common items, but on the right build you might have a use for it.
- For every ~100 million gambled, I expect to get one "heck yeah" item, such as a rare ring with 4 good mods in combo (example: FCR, mana, mf, +stats).
- And for every 250-500 million I expect to gamble a "godly" item, such as the previous rare ring with +life, +resists, and/or pretty high rolls across the board.