Creating non-linear cost functions
When you say non-linear cost function, I assume you aren't referring to the firm's production having non-linear costs, but judging by your example, you rather mean the firm's output prices having weird optimal bundle pricing.

From the abstract of the linked article (Hanson and Martin, 1990):


  " Bundle pricing is a widespread phenomenon. However, despite its importance as a pricing tool, surprisingly little is known about how to find optimal bundle prices. "


So this may be a good place for you to start. For an article that is a little more recent/available online, you could look at extensions of the original work in this area, such as bundle pricing for homogeneous items (Grigoriev et.al., no date), or this article correlated valuations for the items (Chen and Ni, 2017).