The most valuable silk rugs are 100% silk from the warp and weft to the pile. Because of their silk foundation there can be more warp strings (visible as tassels or fringes at the ends of the rug) across with width - meaning more knots can be tied in a given area. With the more expensive materials coupled with finer knot counts comes more skilled artisans.
A typical Persian or Oriental rugs made in wool might have between 100 and 300 knots per square inch - a typical 100% silk rug on the other hand would have between 300 and 600 KPSI. As a result, weaving the rug will require 3-4 times the amount of work which becomes more skilled the more intricate the pattern; meaning at least 3-4 times the price. However, silk rugs are well worth the investment. Not only are they beautiful works of art that can be admired for generations to come but some of the most expensive rugs ever sold at public auction are silk.
Quality silk rugs are robust pieces, but cleaning should only be carried out by a trained professional. This brings up another problem with many art-silk rayon rugs, there are many horror stories of heavy dye running in synthetic rayon carpets during washing. Unfortunately many people do not know their 'silk' carpet is in fact artifical silk until after this happens. It is difficult to wash a silk rug but even more so for an art-silk replica. While it is possible to carefully clean your own wool rug, doing so with a silk rug is definitely not recommended.