In general the cost in any sense of introducing a new variable is trivial or nothing so go wild. Using appropriate variable names is a large part of good coding technique. Having a variable change its stripes easily leads to hard to understand code. The issue is added "cognitive load" - the reader needs to remember more stuff to understand the code.

Another thing to think about is how does changing the meaning of a variable affect debugging? If you introduce a new variable it means you have both versions available for inspection in a debugger at the same time so it can be much easier to see where unexpected results were introduced and why. For this reason I often break down complex expressions into multiple statements with appropriately named variables holding intermediate results. It makes writing, debugging and maintaining the code easier.