Toms hardware already has a number of builds they've done at different budgets, I would start by looking at those builds that are closest to what you're looking to spend. Then, I would look it up against their best ____ for the money articles. So if the build was 5 months ago and they spent $170 on a CPU or something, look up what the current best CPU is for that amount of money and then make adjustments.
pcpartpicker.com also has a list of completed builds, you can narrow the results using a slider scale. I would narrow the results to your budget range and see what other people have been able to achieve recently. You can also post questions to them and ask them how they like the performance of their system.