$300-400? That sounds like a lot! I checked 2 vets in my area, one was $130 and one was $150 for neutering. Mine will be neutered as soon as he turned 6 months. I can't stand the "male behavior" AND, his "boys" are very visible and not pretty to look at at all.
The health benefits are just a bonus. I would never have an unaltered dog, it is too much trouble.