On a budget, St Peter's Dome Climb (Cupola) still earns its price.
€8 (stairs) / €10 (elevator + stairs)
The payoff is huge: an interior balcony looking down into the basilica, then a tight, leaning spiral to a terrace with the whole city at your feet. The final stretch is narrow and not for the claustrophobic, and the lower elevator only saves you the first portion.