That's the power of exponential. Let's do a bit of math. Each bid will increases the player value by 1%.
400T ~ (1.01)^400 ~ 53 times of player value
500T ~ (1.01)^500 ~ 144 times of player value
600T ~ (1.01)^600 ~ 390 times of player value
So if he used a sub-account to buy a player (value ard ~20M) from his main account buy keep bidding, he needed ard 500T to achieve 2B+. 500T is just around 50USD. With the last promotion, 500T is just around 25USD. 25 or 50USD is not a very bid amount of money. In addition, Vietnam is one of the countries which are craziest for football in Asia I would say. So spending 25-50USD is not a very big deal