There's no reason to make one. The main purpose of a starter is to reproduce more cells. With dry yeast there is already more than enough cells to ferment a 5 gallon batch of beer. Just go ahead and pitch.
Yes, if you need more yeast just buy another packet with dry. The manufacturers prep the dry yeast so it is ready to go after rehydration. Making a starter with it would undo some of that preparation.
A quick search will yield lots of posts with some detailed explanations of why not to make starters for dry yeast.
The bottom line is that your yeast will perform best if simply re-hydrated then pitched immediately. It is also often cheaper to simply buy more dry yeast if you need a higher pitch rate than it is to build a starter to make more yeast.