I'm sorry, but you have a misunderstanding of basic economics. If you gather materials out in the world, they are not 'free.' There is an opportunity cost. If you *could have* sold them on the GTN for a certain amount, that is their value. Thus, it does not matter how he acquires those mats. The cost associated with making an item is based inherently on the opportunity cost of the requisite materials.
The OP's concern is a legitimate one. The effective cost (and I'm sure it varies wildly from server to server) of consumables is very, very high.
And selling those materials has an opportunity cost of not using them to create grenades, or in other words not having grenades.
Thus, it does not matter how much money he could make acquiring those mats himself.
The cost here is grenades vs credits vs time. The fact that credits and time are interchangeable is meaningless because it excludes the fact that he obviously values access to grenades.
(And yes, that was an oversimplification to get the point across. I don't feel like typing out a bloody thesis at 5 AM thank you.)