In-game bells have inflated in a linear manner since the release of the game. Initially, the conversion rates on the site were 100TBT for 500,000 in-game bells. This is because the in-game bells were precious and everyone needed the money to upgrade their houses, build public works projects, buy items, etc. Now days, the conversion rates are something along the lines of 100TBT for 20,000,000 in-game bells. This is the result of a game that is two years old. Everyone has all the stuff they want and there is no longer any practical use for the currency in the game. TBT hasn't inflated, the in game currency has. This will be fixed when a new animal crossing game comes out since you aren't able to transfer bells from past installments to new ones. The conversion rate will reset.