So buy the 5th edition. One of the perks of being able to self learn is that these sorts of revenue models don't impact you :-)
Edit: I've used Gallian. Not sure what's in the 7th edition but the 5th is perfectly good.
If it is like other edition changes, you can expect about four typo-fixes, a new sticker on the shrink wrap, a 7-10% price increase, and a single-use code for online tools that make the professor's life easier. Every third edition, a "new and expanded" final chapter and an intro change, and a 20% price bump.
I used to date a textbook marketing manager. It was a joke internally, too.
You forgot the part where you rearrange the questions so that students have to get the latest edition to be able to get the correct question 2 on their assignment.
I used to read textbooks for Reading For the Blind and Dyslexic, and in a number of cases I found that the solutions in the back of the book were not for the same exercises in the front. Somebody had rearranged the exercises, but not the solutions.
Yeah, Stewart Calculus is $285 but you use it for 3 semesters. I still have my copy.