Inflation
Also said as Mudflation, Runaway prices, Hyperinflation
Prices rise because there is more money around, not because anything got better. In a game with no sinks it is almost guaranteed, since players keep producing money and rarely destroy it.
Veterans shrug at prices that lock a new player out for good, because the starting gold that once bought a sword now buys a loaf of bread. Eventually players stop using the official money.
Say it
“Sword prices have tripled in a month. Don't just raise the shop price: add a 5% tax on auction sales that removes the money, and a 2,000-gold mount for the people holding the most.”
Where you’ve seen it
In Diablo II gold was so plentiful that players stopped using it, and traded in Stone of Jordan rings instead. Then item duplication flooded the servers with those rings as well, and they lost their value too.
Not to be confused with power creep
Inflation is too much money chasing the same goods. Power creep is the goods themselves getting stronger. They often arrive together and they have different fixes.