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.
Optimising for fewest key strokes only makes sense transmitting to Pluto or beyond