As written, Nanite Investment is only equal to the price (+ optional 10%) of the last item broken down. Mechanically, this seems odd, as I can theoretically reduce my Nanite Investment by breaking down an item of lesser value; there is no reason to reduce one's Nanite Investment, so the ability to do so is bizarre.
From a less strictly mechanical standpoint, it seems like breaking down an object should add to one's Nanite Investment rather than replace it, as the process is described as breaking down the item into UPBs to add to be used as raw material.
Making it so that breaking down items adds to Nanite Investment instead of replacing it seems like it would make more sense from a mechanical and flavor standpoint. This would also help keep Gear Array forms at an equal level as the Nanocyte levels up by allowing the Nanite Investment to reach a point that it is possible for the Nanocyte to select forms equal to its new level. There is already a maximum item level for forms (Nanocyte level+1), so already having a high Nanite Investment at selection would not allow players to acquire unreasonably leveled items for their Gear Array forms.