Learn how to make starters: It will save money and ensure you have plenty of yeast to pitch.
Once you do it once, you will find how easy it is.
Don't worry about the size of the starter, just make one of any convenient size. A 1 liter starter is better than none, a 2 liter (half gallon) is better.
Any starter is better than none. Once you get confidence in making starters, then you can start refining the size. Personally, I give very little attention to yeast calculators. They get you in the ball park, but there are so many variables, that +/-50% will generally makes no difference. I would say +/- 100%, but that would theoretically bring in the possibility of pitching no yeast.