to clarify what lordxia said, you gain a stackable bonus. If you research refined economics twice, you'll have +20% gold. The total gold bonus increases by 10% each time. If you research it 10 times, you'll get double gold (theoretically. usually there's other bonuses though)
All the bonuses in this game are worked out from the flat values first, and then multiplying with the percentage bonuses stacking additively.
If you have a town which contains a merchant (+2 gold) and a market (+4) you'll have 6 base gold. If you have a 20% gold bonus from the town level, and you've researched refined economics 10x (+100%) then you gain a 120% bonus on your 6 base gold, which gives a total of 13.2 gold per turn from that town.
Also to confuse things fartherr, there's the tax rate, which afaik is the only modifier that stacks multiplicatively. At the basic tax rate of 40% (0.4) the actual gold that town would give is 5.28 per turn (13.2 * 0.4)
that's probably more information than you wanted 
these refined techs are basically a research sink for when you've gathered up everything more valuable. Most of them aren't particularly great for gains vs time spent. The most notable one is the refined mana because the bonus (25%) is much larger, and also because mana has few, if any other percentage modifiers, so it's more likely to make it's full impact. the food one is also good since extra food can be hard to come by, especially in 0 essence cities.
