First of all, thank you @glengarry224 for making this test. I know many of us have been wondering but not doing the actual research. Even if this is over a year old, nearly two, I think this still works the same.
The second, in which case the friendlies is a matchmaking where you can boost the morale - but it will probably have a negative effect on your economy because you'll want to play against weak teams. I have always made the worst economic decision according to this when i play friendlies at home against real weak teams..
This is a difficult balance. You want to have a good economy and maybe struggle to compete in your league and therefore you'll need to arrange some friendlies mid-season(if you have that in your league) so you can, as I said before, boost your morale. If you arrange a friendly home against a weak team, you'll have a negative outcome. The same can apply on away-games against weak teams.
So.. the balance is - play against 2-3 better and bigger teams away in the beginning of the season to save up money for the rest of the season so you can play against weak teams before the first league games and/or in the middle of the season.