Our guide in Agra took us to the Marble Cottage. As part of the sales pitch, we received a demonstration of the pietra dura production process and an explanation about the various gems involved.
We ended up buying an octagonal 14" white marble pietra dura table top. We paid 30,600 rupees for this, including insurance and shipping. I can't really judge whether this was a good price or not, but from online research I would say that it is at least within a reasonable range. In any case, we found the piece worth it since it is a lifelong souvenir of our trip to India.
The price of these pieces is mainly determined by the number of pieces that the inlaid pattern consists of - so a large variation of prices even for the same size of marble is possible.
Mr Azhar, as mentioned by other reviewers, treated us with respect and patience, and we appreciated his effort in helping to find us a piece that suited our taste and budget.
The shipping (FedEx I believe) was timely and the piece arrived at our home address without any damage or trouble.
I would recommend this shop based on our experiences, with the caveat that I really don't have any comparative framework to judge this by.