On a budget, Catacombs (Appian Way) still earns its price.
€10 per catacomb
An eerie, atmospheric counterpoint to the marble grandeur above — miles of carved tunnels and tombs toured by guided lamplight. Note they're three distinct sites: San Callisto is the largest and most popular, Domitilla the only one with an underground basilica and usually the quietest, San Sebastiano the most central of the trio. All sit out along the Via Appia Antica, reached by bus (no tram), so it's a half-day commitment — but it's history you can't get anywhere else.