In this article the budget values examined are $625, $1,250 and $2,500. These are pretty typical costs for build your own PC’s depending on the level of performance that your looking for. I recomend that you read the whole article but here is the conclusions of the article author;

As we’d hoped, performance scales directly to dollars spent, but not proportionally. Performance increases are almost linear whereas build-cost increases are exponential. Doubling our $625 PC budget produced a 78% performance gain in the $1,250 PC, but doubling the $1,250 budget gave our $2,500 PC only a measly 13% gain. Overclocking helped the $2,500 PC more than the rest, but  the value of high-end systems is always somewhat questionable.