I couldn’t imagine flying to Europe just to see a car museum, but it’s your trip. If you have time, take in some other sights, cities, or towns to get more bang for your buck. If you could fly there now, you could get good airfare prices, but when we’ll be able to travel again, prices will no doubt go up. From the east coast during normal times, prices vary significantly depending on a number of variables: airport, connections, airline, time of day, day of the week, which day you’ll return, class ( basic economy, economy, business, first), time of year, etc. About the only discount you might possibly be able to get is a senior/pensioner discount. At the Porsche museum that would save you 5€, but the website doesn’t give a qualifying age to get the discount. Normal adult entrance fee is 10€. In addition to the airfare, don’t forget to include what your lodging and other transportation costs might be. Again, plenty of variables. In short, your question is impossible to answer, especially since nobody can travel anyway.
While in Stuttgart you may as well visit the Mercedes museum.