You can easily get the true value without the speech perks ( accounting for 100 speech skill).
- Mask of Clavicus Vile, a deadric (daedric?) artifact -> 20% better
prices (also known as Fortify Barter - FB)
- Thieves guild helmets
- 10% nooby ones
- 15% better set (I think)
- Cicero breast piece -> 20% better prices
- Amulet of Zenitar -> 10% better prices
- An necklace of another deity (which I don't remember) - 10%. I found it with my first character ever
- Sometimes you can find necklaces in general stores with FB up to 20%
With all the best items above you can get 60%+50% from 100 speech, that gets you 110%. I'm not sure if you can actually get more than the true value for items but you could always try.
I'm fairly sure I didn't listed all FB equipment in the game.
Now say can't get all the equipment listed abover or, for RP reasons chose not to, then there are FB potions; which would be "random of haggling" and the higest I've seen was either 30 or 40% in the game.
Then there's the fact that many skills can be used to make large amount of cash such as:
- Normal Enchanting
- Alchemy (easiest I think)
- Blacksmith ( after the first 60 levels it starts paying off itself I think)
For those shadey characters, you can pickpocket everyone in Riften. If you get caught - and you're in the Thieves Guild - you only have to pay half of the bounty normally is.
And guys if you're having trouble with money try doing a bunch of petty quests or some factions Quest line. Faction Quest lines are always very rewarding.