Basilica di San Marco is free to enter, but it contains several small museums that do cost money. The nave is beautiful, but it is not the main reason to enter. Nor, I'm afraid, is the treasury. If you're me, then your girlfriend is talking loudly about how this artifact and that were looted from Constantinople, and as an avid Byzantinist, she is still holding a grudge. The treasury also costs a few Euros to enter.
So why wait in the long line, you ask? It is so that you can climb the narrow staircase to the balconies. Here there are several informative exhibits, the original four horses (also stolen from Constantinople!), and the exit to the balcony overlooking Piazza San Marco. Step aside, take a deep breath, and soak in the gorgeous view! You can hang out up here for as long as you like, and it's very romantic! I spotted many couples kissing passionately in broad daylight. We snuck around to the side to sneak a quiet kiss, not being exhibitionists.
Note: Filming and photography are prohibited. Also, visitors are required to dress respectfully and talk quietly while within this place of worship.
Free guided tours are offered throughout the day.