I suppose cost depends on the city. The one we took was free. You just gave him whatever you felt it was worth.
You could probably hack one together if you had an old bike you could test on. Have any welding skills? Would make a good HOWTO thread.
I am functionally retarded when it comes to DIY. My brother's a welder though, so maybe if I ask him nicely (buy him beer) He'll draw somthing up and put it together. He lovs my daughter and wife so it's a possibility. I'll post pics if I can, he's got one of them fancy Camera phones.
I did it for work in London last year and it was not as hard as some people think. I made good money and had fun. It would be good for your family but hard on your knees when you do it for a living.
I have been told to make a proper rickshaw you need to build a jig to hold the frame in place while you weld the bike and if you dont the bike will not be balanced and will not ride right but if it is just for fun it may not matter.
We are a brand new hungarian rickshaw manufacturer and have just finished our new models and would like to introduce them to you. Unique shape, composit materials, 60 kg, TÜV certification. http://aeroplastico.hu/riksa02.html