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&...
The expression "Just him and me" doesn't contain a verb, so questions of whether something is subject of the verb or not are unanswerable.
The basic form in English is the object form. In reduced expressions where there is no verb, we use the object form.
Mum: Who wants some ice cream?
(but Child: "I want so ice cream", or &...