Pricing - And Conclusion
Intel's current Core i5-2500K, the high end of the line, is priced at $219, while the entry-level Core i5-2300 is priced at $184. These are expensive prices for processors, but the performance justifies the price. In fact, on a price-for-performance basis, these products are bargains.
However, buying one of these new processors is not as easy as bringing one home and sticking it in to your Core 2 Duo system. Intel has changed sockets since then, so you'll need to buy a new motherboard with a LGA 1155 socket to fit your second-generation Core i5 processor.
In addition to this, the RAM standard has shifted over the last few years. When Core 2 Duo was popular most systems used DDR2 RAM. Today, however, DDR3 RAM is required. This will add another $40 to $120 dollars to your build depending on the amount of RAM you'd like to have in your PC.
When the processor, motherboard and RAM needs are covered, you're looking at a price tag of at least $400. If you choose an expensive motherboard or you'd like to buy 8GB of RAM, you'll be looking at a price closer to $600.
That's a substantial barrier, and it's the reason why even a die-hard geek like myself has yet to upgrade. If you have the money, however, the upgrade will likely be worthwhile. Your new Core i5 will make your system feel brand new compared to your Core 2 Duo.