It depends on what you want to say.
In this context "it can add up" means "Over the course of time, this is going to start to get more expensive than we would like."
Contrarily, "it can't add up" would be used if you want to say "It must not be allowed to get as expensive as that."
Alternatively, "it can't add up" can also be used in the context "it doesn't add up", which means "it doesn't make (financial) sense".
Or, it means "if you compare it against (some figure, or budget allocation), the numbers are inconsistent (that is, the outgoing is greater than what is allocated)."
On balance, if it were me saying this sentence, I would expect to use "it can add up", and I expect that is probably the correct thing to say here.