Brief history: The "Nepali" carpet industry started when there was a plan to provide refugees from Tibet fleeing Chinese rule with a vocation in the 1960's. The Tibetans who came brought with them the expertise of carpetmaking to the Jawalakhel Tibetan Refugee Camp, and from there carpet-making was born into an industry for Tibetans and Nepalis alike. A handmade carpet, 4 x 6 ft, can take up to 8 weeks to make. Child labor is a problem in the industry, like many in the economic south.
There are 60-, 80-, and 100-knot carpets available. The numbers refer to the knots per square inch. As I learned in Tibet, check the bottom very carefully for imprecisions in the weaving, to weed out "machine-made" carpets. This is especially tough for the 100-knot carpets. There are three places I found to buy the carpets - Thamel, Boudhanath, and around the Jawalakhel Tibetan Refugee Camp (500m from the Zoo). The actual Carpet Factory at the camp is expensive, but check the stores around there for cheaper carpets. The best prices I found were around this area, at 5300 RMB for a 4 x 6 m 60-knot (Khamsum Carpets, Jawalakhel and Boudhanath, tel: 012334984). However, you may be able to find cheaper at a supplier (12000 RMB for a 2 x 3 m was bought by my Nepali friend two years ago), but finding the supplier will be tough. Thamel stores were on average 50% more expensive, except for "sale" carpets at Thamel House. Be aware that your "Tibetan" carpet is probably made from Nepali labor, unless you're looking at a very fancy one.
Interesting note: Did you know that many "Tibetan Carpets" made in Tibet are actually from Nepal? According to the Khamsum Carpets shopowner (and he could be lying), the main three purchasers of carpets are from Tibetan businessmen within Tibet, Americans, and Koreans. He stated that although the wool is from Tibet (this could be a lie), the infastructure for washing and dying the wool is in Nepal, as well as cheap(er) labor. Wool is shipped across the border, and carpets are bought by the truck-load back. Based on the costs I saw, this makes sense:
4.5 x 6.5 (ft) 60-knot carpet bought in Lhasa, August 2007: 1250 RMB (USD $166)
4 x 6 (ft) 60-knot carpet bought in Nepal, October 2008: 5300 NPR (USD $72)