On a budget, skip Acropolis Combined Ticket (7 sites) — the spend outweighs the payoff.
€30, valid 5 days
If you intend to see three or more of the central ancient sites it pays for itself quickly and lets you skip several ticket queues — straightforward value for the ruins-completist. The catch: it doesn't include the Acropolis Museum (separate ticket), and a casual visitor doing only the Acropolis is better off with the single ticket.