The TDROP mining reward depends on two factors. The purpose of incorporating these two factors is to deter washing tradings while incentivize marketplace liquidity:
1) The current listing price compared to the highest selling price in history for this particular NFT. If the listing price is lower than the historical high, the buyer is expected to receive a constant amount of TDROP (currently set to 5 TDROP). If the listing price is higher, the TDROP reward will be proportional to log2(price_increase), i.e. the logarithm of the difference between the listing price and the highest selling price in history. This is captured by the f(v_i) formula shown below.
2) The time elapsed since the last time this NFT was traded. The longer this period, the higher the mining reward. It is similar the "cool-down" or "stamina" concept in video games. This is captured by the g(h_i) formula shown below.
NOTE: only the buyers that purchase with TFUEL gets the TDROP mining reward.
You can read more about TDROP here (more about earning in section 6) - TDROP Whitepaper