I use the CPU benchmark website. This site gives a very good comparison guide against price and performance and you can spot some real bargains. It’s a bit of a balancing act really, you can get processors very cheaply but to be honest they are not going to perform very well.
The very best cutting edge performance will be astronomically overpriced but somewhere below what is the very best available will be several processors that are very reasonably priced at a performance level not too far below the cutting edge ones. It’s like that with quite a few industries in that the very latest and very best is significantly overpriced because there will always be those people with more money than sense that have to have them. So the manufacturers know they can charge a premium, but below those there are some snips to be found. Once you have decided on a processor, also known as a CPU which is short for central processing unit, you will need to find a compatible motherboard.