Shoes for Marathon Training?

I'll be running my first marathon in March. I know I need good shoes but I don't want to spend a lot of money. Should I go to a running store that will measure your foot and watch you run to figure out the type of shoe needed? This store is not cheap and I would need two pairs. My $40 Nikes have given me no problems but I'm only running 3-6 miles everyother day right now. I'm in good health, no knee or foot problems and I would like to stay that way.