I would probably start with Ch6 in his _Les mots et les choses_ 
(1966), on mercantilism, and then bring in some secondary texts by 
economists, e.g.

Jack Amariglio 'The Body, economic discourse, and power: an economist's
introduction to Foucault' (HOPE 1988),

Iara Vigo de Lima Onate 'Paradigms and epistemes in the history of 
economic thought' (SCEME Working Paper No. 8, 2004)

Deleuze's _Foucault_ (1986) is still one of the most perceptive 
general appreciations of his work.

Matthias Klaes