When it comes to accepting friendlies, the general rule seems to be that for upper-tier clubs, anything is better than nothing (for the match exp and/or form boost and/or proficiency boost) and for lower tier clubs, money is key. This is why smaller clubs or rebuilding clubs often offer 0% revenue on their friendlies, and why large teams consistently accept those games - despite being offered no money to do so. Occasionally we see very large teams offering 0% and very small teams offering large percentages, but those are rarer occurrences. If a lot of teams restrict their offer (using "min Pop" or "max Div," to exclude tiny clubs or successful clubs respectively) then you may only have one option when you sign in. If you can't sign in multiple times in a day, or you have a number of teams and want friendlies for all of them, you pretty much take what you can.
However, if you did have a choice, and it comes down purely to the money you'll likely bring in, then there is a simple way of knowing which option to pick.
In the above instance, clicking each team name will take you to their team pages. Here we can see popularity again, but crucially we can also see the stadium size. Popularity affects ticket sales in friendlies quite a lot, more so for the home team, so picking an offer which is a smaller Revenue % from a larger club is sometimes optimal, all other things being equal. However, the important information to consider in this situation is Stadium size. A friendly match played at an Honour league stadium with 32,000 seats can bring in up to ~1,500 G. A friendly in a packed 900 seat stadium can never bring in more than 45 G. In this instance, 5% of 1,500 G would be 80 G for the visitor - while 50% at a 1,050 seat stadium would be little more than 50 G.
For larger clubs, this is minimal. However, over the course of a 31-day season, this is almost 1,000 G less when taking the 50% offer (which won't always be available), so to smaller clubs - it's worth a second or two to pick the optimal choice.Full disclosure: One of the above offers is from my own team. However, I hadn't run the numbers before this point - so I always considered 5% to be generous enough. Now I'm thinking I'll start offering 10% instead of 5%. We all learned something here today.