It could certainly have been in her 'An essay on Marxian Economics', and in another paper called 'The Theory of value reconsidered'. Being injust with many other people who wrote on the subject, I recommend a recent article by Peter Groenewegen, "On Re-reading Joan Robinson?s ?On Re-reading Marx?", ROPE 15(4), 2003. There are all Joan Robinson's relevant text for this matter.

Pablo Bortz.