Do you mean shoulder rig?
Or rail system for a tripod?
What kinds of things do you need to have on it?
And what purpose exactly do you need it to serve?
Because there are MANY different configurations of rigs for different purposes.
Well, that's a pretty tiny budget as I'm sure you know. There are rail systems alone that cost more than that from the big brand names.
But I'm always on a ridiculously tight budget so I've found a few great tools that cost a fraction of what some charge.
Here are a few complete rigs. At his low price they will all have some compromises. Some don't have quick release plates. Some have camera cages made of plastic. All have follow-focus units and matte boxes that will work - but they will fall short of really being complete tools and you'll probably end up spending the money twice to replace them.
Better Follow-Focus:
This is the best inexpensive follow-focus I have found. $175 and it has 2 hard stops - which once you've used, you'll never be satisfied without if you ask me. I have this one and it is build really solid. All metal construction, tight tolerances, and pretty heavy. The quick release is great too.
Better matte box:
I have this one too. Heavy but solid. The flip-aside design allows lens-swapping. Have not found anything that beats it at this price. The video reviews are worth watching - I found them to be exactly accurate.