not sure about Cartier stationery prices per se, but I imagine they're likely to be in-line with the pricing at Smythson or Mrs. John L. Strong, where 50 name-engraved correspondence cards + lined envelopes will start around $500
extras like motif, engraved return address, premium envelope liners, etc will entail additional charges.
lvpiggy is right, personalised engraved stationery from Cartier (or Mrs Strong, Smythson, Piccolo Press in Scotland or Wren Press in London) will cost ~US$500 for a set of cards. There is some variation in price depending on whether you choose a monogram, your name and address, or your name, address and a motif (or whatever other combination you decide on).
From what I understand, personalised stationery from Cartier can be ordered at the 5th Avenue store or from from William Ernest Brown in LA.
William Ernest Brown also stocks ready-to-write Cartier stationery. If you'd be content with ready-to-write rather than personalised, it might be worth dropping them an e-mail or giving them a ring to see if they can provide you with current prices.