Really, it all depends on what year of MacBook you have. Yes, getting a recent model logic board is expensive. And, there are many places that sell refurbished logic boards with faster components than our current model. I suggest doing a thorough search of components for a DIY setup if you're out of warranty, don't want to pay for a new machine, and have a knack for unscrewing the cover off and unplugging some wires. Ifixit has some great tutorials.
Actually, it's very economical. You can replace the logic board for roughly $200 along with your processor. Drive your computer into the ground, then replace the logic board with 8gb and it's like having a brand new machine.