Upgrade Items and Heroes without Bankrupting

First we need a good source of gold, we could get that from: So to get regular supply of gold, its advisable to buy Adventure Keys 2-5 times everyday since its is cheaper compared to other keys.
 * Adventure Mode - this is the main source, by spending 300 keys on last levels of very hard mode we could get ~500.000 gold
 * 2× gold event - when there is scheduled gold event, save diary key buys for that event
 * Challenge (~25.000 per day - 5 tries)
 * Tower (not recommended, but could get (~80.000 per day)
 * Diary and Normal Achievements (~10.000 per day)

Upgrading Items
The table below is how to upgrade items based on their rank and +X's. For some cases its better to start using 1-4 higher rank items for upgrade then switch back to lower rank. Its advisable to: Table below is for upgrading +5 and higher ☆☆☆☆☆☆ items:
 * Save Wednesday dungeon keys for Thursday to get upg. only items which provides higher percentages for upgrading.
 * Never limit break 5☆, 4☆ and 3☆ items, evolve them.
 * Avoid using 5☆, 4☆ and 3☆ items for upgrade, evolve them.
 * Save junk 6☆ items for dismantle and use them to upgrade useful 6☆'s.

Upgrading Heroes
For Heroes we can use a slightly different approach because its more costly to evolve and we have enhance evolve option - which allows us to get exactly the hero that we desire. We upgrade heroes in a similar way as items, but there are some differences for 5-6☆'s: Further upgrading 5-6☆ heroes: Here is table for leveling up Heroes:
 * Never sacrifice 3☆ for level up or Upgrade, evolve them.
 * Never sacrifice 3-6☆ heroes for leveling up other heroes.
 * Save Friday dungeon keys for Saturday to get more evo stones.
 * Use gems to buy weekly dungeon keys 2 times on Friday, Saturday, Sunday and Monday.
 * Always enhance evolve useful 4☆ heroes.
 * Its advisable to not Limit Break 5-6☆ hero if his/her leader skill is not useful.
 * One choice is to Reincarnate maxed 6☆ hero once you get a better replacement. If the reincarnation is useless, it can be used for upgrades.
 * For maxed 5☆ heroes that are no longer useful, you can either reincarnate them or random evolve to 6☆ if you have spare evo stones.

Exp and Upg formulas
http://shouyou.com.tw/how/2015/0325/400431.html#.V4eyrbgrK7F

(tradictional chinese)