Both WilliamsBrewing and NorthernBrewer have systems that run about $50 and use single use oxygen tanks from the hardware store. I believe those tanks run about $10-15, but should give you at least 30 batches per tank. Williams also offers a system using a refillable tank, but again, the cost of buying the tank isn't included.
Using a pump from the pet store and a stone will get you more than just shaking, but the pump doesn't actually make oxygen. So it's upper end is what the environment has to offer.
It really comes down to why you're wanting a system. If you make a starter, and pitch the correct amount of yeast, you could probably get away with sloshing manually or using a pond pump as the necessary growth is less, so the need for oxygen is less. That's what I do. If you don't want to do a starter, then a bigger system with pure oxygen will probably be better, more oxygen allows for more growth. I'm sure you can do both with no negative effects.
This is just my experience and what I've learned from research, take it for what it's worth.
