I wish I could suggest an inexpensive bike trailer, but I can't. They just aren't safe in the event of an accident. The best models on the market are Burley and Chariot. Both have superior construction, are made of stronger materials, and have strong roll bars. They are so much better and safer than Instep or anything you could get at Toys R Us. REI carries them as do some bike shops.
We have the Chariot Cougar and LOVE it. You buy it as a basic model and then you add the type of attachment you want: bike trailer, cross country skis, hiking harness, jogging stroller or regular stroller. It has a rain cover, bug shield, a huge storage basket and more. You can also buy an infant insert to use with the stroller or jogger set-up.
The Burley is equally good, except when you use it as a stroller the front wheel sticks out way in front and it just seems hazardous to others on a crowded sidewalk.
You can occasionally find used Chariots and Burleys on Craigslist. New you will spend between $300-$500. So again, not cheap, but VERY safe! Good luck and happy biking!