When you spend the money, how you spend it, etc, makes this a fools question.
I spent $30 to get the veterans to 7* to get JTR in October as opposed to waiting 3 months for her. She profitted me well more than $30 worth of stuff just by making me a top 3 str finisher for my guild. Or by making me able to 3* all light side battles, all the mod battles, etc.
I spent $10 on enfys nest to go from 3* to 4* to buy the Marquee pack, to still have her at 5* right now cause I only recently was able to start farming her. Sure, I love her as a toon and she's won me some battles, helped me some when I did that, but clearly far worse ROI compared to vet han and chewie, objectively worst toons.
I used to buy a $20 crystal box every month or so because doing 3 cantina refreshes and 3 regular energy refreshes would out spend my crystal income by like 50-100 crystals a day. That gave me freedom to farm things a little faster, get a full g12 jkr team, that bumped me from top 200 to top 50 in arena, which then changed me from net negative to net positive on crystals a day (that and stopping refreshing cantina as I ran lower on needed toons), so that I don't need to buy the crystals as much/often (and now using my net profits to buy gear or shards or more refreshes here and there).
There's no magic or predescribed way to spend on this game. So no magic $$$ to guarantee success either way