The Recent Average Price, often abbreviated as RAP is what is calculated in trades and an average selling price of each individual limited item.
A limited item's RAP is reflected by how much it sells for on the avatar shop, trades for, and its daily amount of copies sold averagely. A limited can sell for above or below its RAP, both affecting its 'Robux' value. If it sells above its corresponding RAP, then it will increase, if it sells below, then it will decrease; depending how much it is sold for.
In order to find how much an item will raise or lower can easily be interpreted. Firstly, subtract the lowest selling price from the current RAP, and divide that result by 10, then round it to a whole number. When calculated, the number that you receive is estimated to be how much Robux will effectively ascend or descend the limited's RAP when it is sold.
A limited unique hat's RAP is 12,500 Robux
15,000 - 12,500 = 2,500 2,500 ÷ 10 = 250 12,500 + 250 = 12,750
35,000 - 30,000 = 5,000 5,000 ÷ 10 = 500 35,000 - 500 = 34,500