as written, no cost - it's a class feature. Each item is activated with an action equal to the spell involved - which DOES get fiddly if you mix action types in the same item. Presumably there are verbal and somatic components, RAW, as well. Material components are nixed. RAW, that's bad :/ I missed that initially. It does require some RAI reading as written.
@Rymosrac: I missed that. Yes, RAW that's one valid reading. On the other hand, 'any number of spells' - "I put lightning bolt and lightning bolt into it" -> "You already put lightning bolt into it". It's going to end up all 'yo dawg'.
I mean, sure, the building time does limit it quite a bit, too - 4H per effect, and presumably the device must always be made from scratch, and not upgraded with time like magic items.
Okay, let's play...
-that there's a 'no expensive material components' clause and
-the item uses the longest casting time.
Of course, one reading would also be that you need to fulfill all activation clauses, and that it's an extended action to activate the item with multiple effects. Frankly, somatic components feel stupid, while pressing a button or another equiv. of a somatic component is fine.
- let's play somatic component is replaced by a noise generated by the device starting.
So your lightning bolt in lightning bolt in lightning bolt would take a full-round and a standard action to activate.