You do not have to have an agency to help you arrange things in Tibet. You will pay thru your nose using them.
I flew into Lhasa from Chengdu. I will only take you a day to get things -plane ticket, permit- arranged in Chengdu. Most travel agents that service foreigners do that, but one good place to recommend is Sam's G/H. Search for them on the net or Lonely Planet's site. It costs me 1700Y all incl of ticket & permit.
In Lhasa, the best way to go about your trip is to leave notices at guesthouses' notice boards as well as read the notices. Best place to stay- cheap and very good Tibetan atmosphere and set-up is Pentoc G/H. There are heaps of people doing the same thing that you do and normally going the same standard route all the way to Everest BC, and another trip north to Namtso Lake region. Reason is that these are more or less the only places foreigners are allowed to go. The prices are quite standard- I forgot but you can tell from the notices. You normally share with 3 or 4 others in a 4WD. There are a few places around Lhasa (Sera & Ganden Monasteries) which can be visited on a public bus. There are places such as Potala Palace and Jokhang Monastery (a couple of hundred metres from Pentoc GH) which you can visit without any fuss.
We paid 1250Y for a 3D/2N 4WD to Namtso and a detour to Retin Monastery on the way to DrigunTi Monastery (sky burial holy ground).
We also paid 3800Y for a 4D ride to Nepal via Everest BC thru Yamdrok-tso, Shigatse, Gyantse, Tingri Plains etc. You can return from the EBC and it takes another 2 days back to Lhasa. You will have to pay more but not a lot more cos the driver will have to come back to Lhasa anyway.
Please note these prices are 2003 and you share the costs with how ever many in the 4WD.
I used this guy for the 5 days trip and his driver and car was VERY GOOD- highly recommended. KEDUP tel: 6982386
Hope this is useful and have fun. I did (very much).