No, this difference is the surplus value, which is part of the value of the good for Marx. If you remove the surplus value, you get the production cost. Marx does not argue that the "value" is just the production cost, it is certain that in a capitalist economy, which was what he was analyzing, the surplus value is part of the value, otherwise profit and growth would be impossible. Marx repeats in several parts that he considers in the model that all goods are sold exactly by their value, which for all goods, the surplus value is included. But later, Marx acknowledges that in fact, prices for each good cannot be explained solely by value (production cost + that surplus value) and that this simplified model needs to disappear when you take into account a model with several different industries competing.

But I do no not agree exactly with asu_thomas that the labor theory of value is so unimportant for marxism and I think you are right in questioning the affirmation. But I think you mixed a little the value theories from Smith and Ricardo and from Marx in your interpretation (the first two are the ones that do not consider this "surplus value" in the value and try to explain the capitalist profit as coming somehow from the labor of the capitalist or from other effects in the market and environment). And I also do not agree that the marxist theory of value is debunked and so easily dismissed. Certainly it is not a mainstream theory, but there are still marxist economists nowadays between the heterodox ones.