Effects
As its name suggests, the Repel item repels wild Pokemon, preventing them from attacking you when you wander through unpopulated areas. Repel comes in three strengths: the standard Repel prevents random encounters for 100 steps, Super Repel repels Pokemon for 200 steps and Max Repel wards off wild Pokemon for 250 steps.
Item Description
Repel items are placed in your bag in the "Items" section. They all look like aerosol spray cans, but they're different colors: Repel is green, Super Repel is silver and Max Repel is gold.
Cost
All strengths of Repel are sold at Poke Marts in every city and town in the game, although you may also find Repels hidden in Pokeballs. Repel costs 350 Pokemon Dollars, Super Repel costs 500 Pokemon Dollars and Max Repel costs 700 Pokemon Dollars. Of the three, Super Repel is the most cost-effective: If you're walking a total of 1,000 steps, it would cost 2,500 Pokemon Dollars using Super Repels versus 2,800 when using Max Repels and 3,500 with basic Repels.
Considerations
The item descriptions indicate that Repel items ward off weak Pokemon; in this case, "weak" refers to Pokemon that are of a lower level than your lead Pokemon. This Pokemon appears in the top left position in the Pokemon selection screen and is the first Pokemon to fight when you get into a battle. Thus, if you activate a Repel with a level 14 Pokemon in the lead position, you'll prevent encounters with Pokemon levels 13 and below for the duration. For your Repels to be most effective, you should place your highest-level Pokemon in the lead position.
The Repel Trick
Repel items can also help you to capture high-level Pokemon. For example, if you know that a certain Pokemon appears in a particular area and its level is usually between 23 and 25, place a level-25 Pokemon at the front of your party and activate a Repel. The item will block all Pokemon levels 24 and lower from attacking you, increasing your chances of encountering the desired level-25 Pokemon.