The cost relative to the batch is just comparing the cost estimates of each statement within the batch. That's all. It's not itself a measure as much as it's a comparison of other measures. As a comparison, it's useful. As a number, it's meaningless without then comparing the other numbers.

The most important thing to remember when dealing with execution plans is that the cost estimates, regardless of an actual plan or an estimated plan, are always estimated values. They are never real values. While they internally reflect mathematical models that simulate I/O and CPU, they are not measures of I/O and CPU. All that said, they are the numbers we get, so we're going to use them.