Earlier this year, the cost to purchase coins in Pokemon Go increased in several countries as a result of adjustments to the exchange rate compared to the US dollar. However, the game recently reduced the rate of its 100 PokeCoin bundle back down to its original prices. This price drop only affects the 100 Coin bundle. The larger bundles of Coins still remain at their adjusted higher rates.
The price drop went into effect in the UK, Australia, India, France, Poland, Mexico, Canada, Germany, and several other European countries.Since the price drop only impacted one set of coins, this doesn't appear to be a currency rate readjustment. It's unclear if this is a permanent price drop or just a temporary drop to get people spending money during the game's Adventure Week event. Pokemon Go reduced the price of Poke Balls by 50% to coincide with the event, but they didn't advertise any price drops for coins or other events.
Notably, the drop in coin didn't go into effect in either the United States or Japan, which are arguably Pokemon Go's two biggest markets.
There's no telling how long the price drop will last, so you might want to take advantage of the discount while it's still in place.