Money Glitch
     Begin  a  mission, any mission. When your motherships appear, and
you  go  to  the  select mecha screen, press X. Unequip all parts from
mecha,  and  sell them. Then, begin the mission, and fail. You will be
taken  to  the pre-mission screen, with all the money from selling the
parts,  and  the parts you sold. Repeat as desired for infinite money.
It's  possible  upgrade  every unit to the max this way as soon as you
obtain them.