You need to ignore the ghz rating as it went out the window years ago. These days it is more about cores, and hyperthreading and instructions per clock cycle, etc. In other words, newer tech of the i5-2500 vs. the older tech of the others.
Right now, anything other than socket 1155 or socket 2011 on the Intel side is essentially a "dead platform". This means you can still buy it, but you will not have much of an upgrade path, if any at all.
It also depends on what you want to do with the machine. It it strictly gaming? Only for heavy video or photo processing? CAD, 3D modeling? All these will determine the best processor for you along with your budget.
As to older chips being more expensive, it usually happens that the new stuff is less expensive and faster than the old. Just the nature of the beast. Why some vendors do not mark down the old in price and move them out has always been beyond me.
You can always view Tom's charts and others around the web to see where a particular CPU, or GPU for that matter, rate in the scheme of things.