Gross margin - dense vs. sparse block evaluation mode in MDX

Gross margin (also known as Gross profit margin or Gross profit rate) is defined as (Revenue – Cost of Sales)/Revenue. In terms of Adventure Works sample database we can write this in MDX as

[Gross Margin] = ([Measures].[Internet Sales Amount] - [Measures].[Internet Total Product Cost]) / [Measures].[Internet Sales Amount];

While this expression is simple enough, it might be tempting to try to optimize it. If we look at the evaluation tree for this expression – we will see 5 nodes in it: Fetching values for Internet Sales Amount twice – because it appears twice in the expression, fetching values for Total Cost, one minus operation and one division operation. Accessing Internet Sales Amount twice is not really a problem, because second access is going to come from cache anyway, but one might think that removing this extra operation would still improve thing, if only a little.