Rome is expensive but can be done affordably. Budget travelers spending €65-85/day can cover a hostel near Termini (€20-35/dorm), pizza al taglio and supplì for lunch (€5-8), trattoria dinner (€15-25 with house wine), and walking/bus transport. Vatican Museums and Colosseum combined cost about €35 — pre-book to avoid paying a reseller premium.
Free experiences abound: the Pantheon exterior, all the major piazzas, the Jewish Ghetto, Trastevere's streets, Villa Borghese park, and stunning churches (Santa Maria Maggiore, San Clemente's underground levels). First Sunday of every month: state museums including the Colosseum are free — arrive at opening to manage crowds.