I don't think it actually matters in this equation (the amount of money). The amount is all perspective. It has to o many variables to make a coherent conclusion. What you get out of it depends on your personality, the game itself, the other items and your personal life/situation to name a few. Some people think $2 is a great price for ac scratches, others think they get their value with $3 mags. If we are talking about this game, $9 could seem like a droplet in a lake, but to others it could just seem like a waste of money. You and I don't know others situation, some family get by with less than that a day. So $9 might be a lot for something that wont take you very far, or it might be enough to keep you happy and interested in the game.
But to generalize the answer, it really depends on you. Imho, I don't think $9 for a booster is really worth it, considering you can get other boosters through the game, and because there is still so much more content missing you are cheating yourself out of getting other drops that are not in the game yet but will also be used on those same boosters if you wanted to play it. Again it is your money, I am not telling you how to spend it, I am just saying I stopped worrying about percent drops a long time ago. Doesn't mean I don't put the same amount of effort as you do, it just means I know how bad my luck is and this wont help it, so I'll out more effort to make up for the luck.