I keep playing this game, b/c I want every weapon in the game as a lvl 10. It drives me to want to play more and more. In doing so, I've come across a strategy for properly using the store for weapon upgrades. By doing this, you will get every weapon as a lvl 10 as cheaply as possible. Except N7's as they are only given out as reward packs every 2 weeks.
1. Don't overpay for lower level weapons. I don't adhere to the policy of always buying PSP. This only makes sense when you have every uncommon weapon as a lvl 10. If you do, then you are very likely to get a lower lvl weapon in a PSP, and you are way overpaying for it. Only buy PSP when you have all uncommons as a lvl 10.
2. Start out by getting all COMMON weapons to lvl 10 by only purchasing RECRUIT packs. (very easy and cheap to do).
3. Get all of your UNCOMMON weapons to lvl 10 by purchasing only VETERAN packs. (still pretty easy and realtively cheap to do)
4. Now buy everything with PSP. You get more bang for the buck than a SP. Example: For 600k, you can either buy 10 SP = 10 RARES or 6 PSP = 12 RARES. Yes you get more rockets, medigels, etc. But if you use my theory above, I filled out every equipent with 255, so I don't need them. And yes, you'll get more equipment, but ask yourself, do you care more about equipment, or weapons? For me, it's all about the "pop-pop!"
I've done this theory, and I have every RARE and below as a lvl 10. Most of my ULTRA-RARES are lvl 4 and above. I have every weapon in the game except for the stupid BW and Javelin. But I feel confident that I will have them soon. Also, N7 weapons are only given for reward packs, so there's no real strategy there except to always complete the weekend challenges.
For anyone interested, I have an excel sheet that lists every weapon according to class, and you simply add the lvl beside it, and it tallys when you've completed a section (commons, uncommons, etc). So you would know when to purchase Recruit packs, Veterans, and PSP. Send me a message if you would like it.