If you can do the graphics yourself, you can usually find a sign company that'll do signs for around $20 - $30 each for something around 18"x12". That's really not too bad for very nice signs.
But...
If you want it really cheap and do the graphics yourself, you can get Staples to print and laminate them in 10 mil laminate (which is pretty ridiculously thick) for around $2.50 for each 8.5"x11" sign. I imagine it would fade much more quickly than the proper signs, but it's also about a tenth of the cost. This laminate is much thicker than you're average stuff, I was very surprised by it.