So, where does the Mother get all of the money from in the film? Her job doesn't come across as a high paying one, they're passing off Chinese herbs as the real deal, and her boss kind of threatens her over doing acupuncture on the side without a license so she can make some extra scratch. After her son has to pay for the mirror she mentions that they have no money, but somehow has enough money to hire that lawyer at least for a short while. Jin-tae also demands she pay him and she gives him a little bit of money, saying she was saving it up to pay off their debts and it's all she has, and then shortly after that manages to hand him a wad of bills to get information out of those two guys, which was apparently enough for him to buy a new car.
in the case with the comment after the mirror and paying Jin-tae for the "betrayal" she could have just been not speaking literally i.e. she still had savings left but it wasn't something she could necessarily look at as freely expendable income. you could also choose to believe she never got around to actually paying for the mirror, taken out a loan, etc. Jin-tae could have also had some money before so it wasn't like the amount he received from Mother actually paid off the whole thing. maybe he had other means. like he did steal the golf club earlier. maybe he kept Ah-jung's cellphone and got around to blackmailing the men whose pictures were in there. who knows?
as a side note, for a while, the largest denomination of SK currency was the 10,000 Won which you could think of as equivalent to $10 US. The 50,000 Won note (~$50 USD) was introduced in 2009. so that thick wad of bills probably wasn't as much as it might have seemed.