There's so many out there that sell their tutors to upgrade. It's a bad strategy and here's why. 1. It weakens you and makes you more vulnerable to attacks. Having good tutors makes you stronger 2. You make less money per hit. More tutors means more cash per hit. There is a cap, however. The cap is based on your size/strength so it varies from player to player. The amount of cash gained from each hit depends on the size of your opponent. Without tutors I my plunder is cut in half. Notice how my plunder alone is significantly less by itself then when combined with my tutor bonus? And, that, my friends is why you shouldn't sell tutors.
But then upgrades become slower because you're making less. The coat of dorms/crew increases the higher you go. You need that extra plunder to grow quickly.
Sophie you should always keep a good stock of tutors, then save up extra $ for upgrades. Dev is 100% correct. If you sell all your tutors to upgrade, then make crappy money trying to restock again, it will take you a lot longer to save up enough money for your next upgrade.
My advice for ppl who don't like or just can't possibly save a cool 1bil is to just get double your dorm cost this helps as you can upgrade your crew to lvl 4 straight away, afford a stronger crew and you will have enough money for a decent tutor afterwards