To fully upgrade a character, you first need to recruit it; To do this, you must get a small amount (10 or 20) of their biometrics (for X-Men characters, X-Genes - completely mechanically equivalent).
Most characters are simply farmable; There are mission you can play that will give you 10 of their biometrics a day, and you'll get them to 6 stars in about 2 months of daily farming - note you can farm them all concurrently. Then you'll need about another month of farming to get them to tier 2 (This is totaling about 900 biometrics), and that's where the vast majority of his character have stopped.
Some are semi-farmable; They can be farmed but either take trickery or an insane amount of patience - we'll assume neither option was chosen by Phil. The alternative is to recruit them (easy, again this takes like 10 or 20 of the stuff) and then use tickets (items that automatically rank-up a character from a rank to the next, or even straight to 6 stars, the maximum, or skip part of/the entire T2 upgrade process) to rank them up; Tickets are given away somewhat regularly, you can rank up a few characters that way every year, but semi-farmable characters are also pretty rare themselves (I think there were maybe 10 total released over the course of the game, several of which were not when Phil quit), so all in all, this likely did not pose much problem to Phil.
Then there are premium characters whose biometrics can only be acquired by paying a subscription (the one for "regular" characters and mutants are different, but that's pretty much irrelevant); You could technically buy a single subscription of each (9.99$ for 600-or-so biometrics total, to spread mostly however you want between characters) to unlock every premium characters and then rank them up solely via tickets, but it's unlikely Phil did that.
That's likely where most of his money went since he has almost all of them upgraded to tier 2; That's about 15$ a pop' assuming no ticket use (which he definitely should have done and maybe even did), and there were about (I checked and guesstimated when he quit without checking exactly) 25 when he quit, let's say 30 for the sake of the argument; That'd be 450$ tops.