Yes, they are legal here.
I've never even heard of Degus until 2 days ago on the internet. Today I visited 2 pet stores, they didn't have any.
If they are so rare, why are they so inexpensive?
They aren't rare everywhere, it depends on the area (and it seems that your area is one of them). Which is also why the price varies.
In some places, I've seen them sell for almost $50. Where I live, you can find them for $10 because we happen to have a breeder in the area. Once I bought a degu for $5 because a store had a surplus of them.
That's pretty much what it comes down to--whether or not you have a breeder nearby.
But like I said, degus are exotic pets. The average pet store isn't going to order them which is why it takes a bit of searching. Since I don't live anywhere near you I can't give you names of breeders or anything (I wish I could), all I can do is offer tips based on my own experiences (degus weren't always so common where I live and it took me quite a while to get my first pair).