Notice the tour guides above are posting messages to beware of fake recommendations. Likewise, I am advising to beware of what appear to be helpful advice but merely more pitches for your business from competitors.
I am not recommending, but you might check out the Lhasa-based, Spinn Cafe, http://www.cafespinn.com/en/tibet/index.html
It makes sense that a tour company based in the relatively isolated country you are visiting might have better access to tourism resources and local money-saving practices and customs. For example, depending on local "police conditions," you might be advised that you are able to wander in Lhasa without a guide and save a couple USD$$, except for required places.
As always, never pay cash or wire transfer in advance to any tour company, always with credit card in case things go wrong along the way. If part of the Spinn Cafe website is not working right, just email Pazu and he will get it back up and running quickly.