Considering that Descartes is the originator of the most famous single phrase in Western Philosophy; 'I Think, Therefore I Am' it is perhaps surprising that his true first editions are only priced in the handful of thousands, rather than tens of thousands.
One of the many reasons for this might perhaps be due to the production and presentation of his books - a fact that he can hardly held accountable for. At the time of printing (the mid seventeenth century) it must be remembered that there were different sensibilities to manuscript printing than there are today. The volumes themselves might appear small, or with too many pages for secure binding. Alternately, they are often compiled together in collective works (so it is customary to find The Meditations… for example, also bound with the Objections which is actually a latter clarification of the earlier work). Book printers at the time, we must remember, might not have been imagining that these volumes would last four hundred years, and be widely sought after. They were printing for what they thought was a very small, select community.
Perhaps another reason why Descartes work is so low priced (comparative to its cultural impact) is due to a certain bias in language appreciation. Descartes published (almost exclusively) in French and Dutch, which was thought to provide a degree of anonymity and to remove the work (slightly) from the attentions of the Catholic Church.
For the serious collector, I would heartily recommend searching for the Discourse of the Method in its 1668 Paris edition. If you can, try to find the 1637 True First, which is published anonymously due to Galileo's conviction by the Catholic Church a couple of years earlier! Other than that very fine piece of history, consider his more scientific works L'Homme (a study of the mechanistic nature of animal biology) or 'Geometria' Descartes study of mathematics, geometry, and physics.
Descartes, Rene. Discours De La Methode Pour bien conduire sa raison, and chercher la verite dans les sciences. Jan Maire, Leiden (1637). Plus La Dioptrique. Les Meteores; Et La Geometrie; Qui Sont Des Essais De Cete Methode; Morocco Paste-Downs With Gilt Rule, By Georges Huser.
Descartes, Rene. Principia Phlosophiae. Lowijs Elzevier, Amsterdam (1644). Bound With Specimina Philosophiae: Seu Dissertatio De Methodo Recte Regendae Rationis,and Veritatis In Scientiis Investigandae: Dioptrice Et Meteora; Illustrated With Numerous In-Text Diagrams.
Descartes, Rene. Musicae Compendium. Gisberti a Zijll and Theodori ab Ackerskijck, Utrecht (1650). With A Woodcut Device On The Title And Numerous Woodcut Diagrams And Figures, Including Music Printing, In The Text.