At first I spent very little on super mechs.
I spent 40 dollars to get gold to fuse my legacy items
Then I did the following
When the update was new you could buy a premium box for 150 tokens and get 2 epics or a epic and a legendary or even 2 legendaries, which I did and I spent 50 dollars on them, to get premium items like a shotgun and a sniper.
Then I started spending my tokens on fuel instead of boxes and I would spam farm bigboy for 20 tokens I was getting 90 fuel and that = 18 epics (and sometimes a legendary) for 20 tokens.
Then they nerfed big boy and premium boxes, boxes now gave 1 item for 75 tokens but I still bought fuel 90 fuel = 9 epics for 20 tokens.
Then they nerfed big boy again and he started giving only crap commons and rares (rarely a epic) so I went back to premium boxes because you could not use the stuff from bigboy to transform now, and I spent around $ 350 to finish my mech which is still not fully finished
I spent probably around 450 dollars maybe 500 since the update.
For some reason the only good stuff I ever get is energy items so I went with a energy mech.