I built a draft cart from scrap lumber in my garage and a set of $20 wheels. I had to buy some sturdy screws, too.
The siwash harness costs $64. Harness
It is helpful to have instructions on how to get started. Dogs get startled by the cart shafts. There is a DVD you can buy, and there used to be a really good carting yahoo group. I gave my DVD away. The information on there can be found online (I think).
If you want a cart big enough for kids (4 wheels?) that could cost you, unless you have good building skills.