You make money from the campaign, which is still a good enough incentive to play it However there have been some good ideas brought up on how to improve the reward of single player compaign.

There is the option to fuse old items though A selling option doesn't work, as ammount total price of the items obtained in the items boxes are normally higher than the item box itself. That means you could just buy an item box, sell those items and make money from it.

Hmm... getting some better item box for each continent cleared or something would be cool.

And I prefer to gain money from missions, instead of useless parts, with which I wouldn't know what to do.