What is a cheap way to get from Nepal, through Tibet and into China?

We are planning to travel next March - May from India to Nepal, and then on into China via Tibet. It seems you can only go through tibet if you have a group visa, so you have to do a group tour, and these are very expensive!!! Does anyone know of a cheap way to travel through Tibet, or details of some cheap group tours to get there? Please let us know any good websites/tour leaders!