Part of the reason we've avoided doing so is that, let's face it, it has the potential to be kind of cheesy. There's much more to a CPU's value proposition than a cold cost-benefit analysis can capture, and in truth, doing such an analysis well can prove rather tricky. That's why you should read our CPU reviews and our system building guides to see what they recommend.

However...

A vocal contingent of our readers has long been asking for a closer look at price-performance issues, and we think we've cooked up some novel ways of expressing that data that may make it feasible. So we've decided to give it a shot.

Fortuitously, AMD and Intel both took an axe to their prices last month, and we recently added Intel's $113 Core 2 Duo E4300 to our constellation of test results, so now seems like a particularly appropriate time to consider performance per dollar. Join us as we look at the value proposition of 16 CPUs, from the Athlon 64 X2 3600+ all the way up to the Core 2 Extreme QX6800, across a wide range of games, applications, and even energy efficiency tests. Some of what we found surprised us, and it may change the way you think about CPU value.

Quantifying CPU value
In theory, quantifying value is easy. We can measure performace quite well, prices are easy to check, and dividing the former by the latter gives you performance per dollar—except it's not quite that simple, for a number of reasons.

First, processors aren't always the only factors affecting performance in a given task or benchmark. Games, for example, tend to favor GPU pixel-pushing horsepower over CPU computational grunt. Memory bandwidth can limit performance, software and operating systems can be wildy inefficient, and don't get us started on the bottlenecking potential of hard disk drives. Then there's the issue of whether software makes the most of a processor's abilities. This one is a particular problem for quad-core systems, since few applications are multithreaded well enough exploit them fully.

And, of course, there's the question of cost. Sure, it's easy to pull from official price lists, but bulk pricing doesn't always track with street prices. Bare processor prices don't take into account overall platform costs, either, or the cost of power consumption on your utility bill.

We've attempted to mitigate some of these issues by providing value analysis for a wide range of applications and processors, so we can draw conclusions based on overall trends rather than just a handful of numbers. We can illustrate which processors offer better value than others and under which circumstances.

Here's a quick run-down of the specifications of the Intel processors we'll be looking at today. We took these prices from Intel's official processor price list, since street prices tend to vary from vendor to vendor and fluctuate without warning.

This is a classic example of CPU price structure. Intel's Core 2 prices ramp up much quicker than key specs like clock speed, cache size, or the number of cores. For instance, the E6300 may have half the number of cores, one quarter the cache, and a 38% lower clock frequency than the flagship QX6800, but it costs 86% less. Or consider the E6600 and Q6600, both of which run at 2.4GHz. The latter is essentially twice the former, but the difference in price is actually close to 2.4 times. Spending more doesn't necessarily get you an equitable boost in computational power, a trend we see continue with AMD's offerings.

AMD's price range dips lower than Intel's, but it also doesn't reach beyond $799. Then again, the Athlon 64 FX-72 and FX-74 require a dual-socket motherboard that currently sells for more than $325, so there's a considerable additional cost associated with that platform.

Here, also, prices ramp up faster than key specs. An FX-72 setup gets you the same clock speed, number of cores, and cache size as a pair of X2 5600+ processors, but it costs more than three times as much. Similarly, the Athlon 64 X2 3600+ gives up 37% of the clock speed and 50% of the cache of the 6000+, but sells for just 30% of the cost. Based on their specs alone, budget chips certainly look to have the best value propositions.

Charting relative value is a little new for us, so we've come up with a couple of ways to express performance per dollar. The first is the easiest: a simple graph depicting the value of a processor's score in a given test—be it in frames per second, or as an encoding rate, or even an arbitrary benchmark score—divided by that processor's price. In some cases, such as with media encoding, we've had to do a little multiplication to avoid generating value scores with too many decimal places to express succinctly. This doesn't taint our results, though; it just makes them easier to read.

Our second tool for evaluating processor value comes in the form of a scatter plot, which looks like so:

Of course, picking a processor isn't typically about what's best as much as what sits in the mythical price-performance sweet spot. To determine that using our scatter plots, you'll want to find the cutoff where either a) performance keeps increasing but starts to cost more and more, or b) performance stops going up significantly—or at all—with price. In a scatter plot like the one above, for instance, the latter would apply. There are exceptions to this rule, though, as we'll see in the next few pages.

The scatter plot might look a little daunting, but it has the advantage of providing an instantaneous look at how price scales with performance. Most of us can afford a processor that costs a little more than the $73 Athlon 64 X2 3600+, so it's helpful to be able to spot the best performing CPU within a given budget.