An enthusiast will spend $1000 for the latest and most powerful CPU. The rest of us are happy with mainstream processors. Exact prices and offerings may vary, but for reference:
For $20 Newegg will sell you a single core AMD 2.2 GHz chip. Four years ago this would have been a top-of-the-line chip, and since then Office, the Web, and even Vista have not become all that more demanding. This processor will not decode Blu-Ray movies on its own, so you get integrated video that takes care of that for the CPU. The cheapest Intel single core chip is $40.
For around $55 you can get an Intel Dual Core Celeron (1.6 GHz) or AMD Athlon (2.7 GHz). The AMD chip is more powerful, but even more important is the fact that you can get much better AMD mainboards in the $50 to $80 range than the Intel systems.
However, around $83 there are modern 45nm Intel dual core systems with 2.5 GHz that are a much better choice, and the extra $18 is not important in the overall cost of the system.
For $105 AMD offers a Quad Core Phenom at 2.3 GHz. You will have to pay $180 for an Intel Quad Core 2.33 GHz.
While a Quad Core system offers extra power for a few applications that can use it, the least expensive quad chips use more power all the time than corresponding dual core systems. This suggests that you go back over the chips looking for a number that is easier to find on a refrigerator than a CPU, how much power will the thing draw on average during a year. CPU chips are rated by the maximum power they will use (and heat they will generate), while actual power use probably depends on the minimum power used by a system that realistically will be mostly idle. Intel generally has an advantage here because the Core 2 family borrowed technology from what was originally a laptop design.
Nehalem/ Core i7 will be interesting in mid 2009 when it begins to play in the mainstream market. Today the CPU chip costs a little more, but the mainboard costs lots, lots more because it is designed for gamers with two or three high powered video cards.