Sorry Qwerty, but your answer is incorrect.
There are 3 items that are tied for the highest price - a whopping 1 million PokeDollars.
Venusaurite, Charizardite X/Y, Blastoisite (Lumiose City, X&Y) - 1 Million PokeDollars. This price goes down with your style points.
Bicycle (Cerulean City, FireRed & LeafGreen) - 1 Million PokeDollars as well. This price is negated with a Bike Pass, giving you it with a 100% discount.
The Slowpoketail sold by the man outside the Union Cave [HeartGold & SoulSilver] is also pretty expensive. In fact, holding 1,000,000 pokedollars was impossible in the game, making it the most expensive item here. | Source: QwertyZoom