I found a big selection for under $50.00 on Ebay...
Dog Strollers | eBay
Some are just strollers, some double as carriers and/or car seats and/or pet beds, so it really depends on what your intended use is.
Also, the first stroller I used was an umbrella stroller ($12.00), easily adaptable for a dog. I wanted to see how Peek a Boo liked riding before spending money on a better stroller.
I just sold mine to my neighbor...