In general, if you want to buy a new, high-end paintball gun, you should plan on spending about $1,000 when you factor in the cost of a loader and an air tank. If you want to spend less, consider getting a previous year's model or a used gun. Many teams use their high-end guns for one year and then sell them before the next season.
If, though, you simply want a higher-end paintball gun that will allow you to compete with anyone, you can probably find one for around $300. The differences between guns that cost over $300 are minimal and you end up spending many hundreds of dollars for very slight performance improvements.