A weekend in Rome can be really affordable.
You can get almost anywhere in the city using the metro or simply walking. The main tourist attractions in Rome fall between the Vatican and the Colosseum. Going from the Vatican to the Colosseum can be accomplished in a one-hour walk. For just 1.50 euros you can get a metro pass and go from the Lepanto (Vatican) stop to the Colosseo (Colosseum) stop. Taking the metro can reduce travel time to 20 minutes on foot and 10 minutes on the metro.
The metro is really easy to navigate with only two different lines that both connect at Termini Station. The order of stops is clearly listed on many signs at Termini so you can find where to transfer lines upon arrival at Termini. The 1.50 euro gets you one metro ride or unlimited bus rides for a 100-minute duration. You can mix this up by taking one metro ride and hopping on a bus for whatever remaining time you have left on your ticket. Whether walking the entire distance or taking the metro, it is really cheap to travel around Rome.
Of course, you wouldn't want to cram your entire visit to Rome into one hour or even one day. There's so much to see at the Vatican, between the basilica and the museums (admission for the museums is free on the first Sunday of the month), and also at the Colosseum. For only 12 euros, you can go inside the Colosseum and walk around Capitoline Hill, the Roman Forum, and Palatine Hill. The original city of Rome was built on top of these hills with the Roman Forum originally serving as a graveyard.
The vast majority of places to see in Rome are free to explore, making it a cheap city to journey through. There are plenty of monuments and churches to see in the main city center of Rome (where the Colosseum is), easily consuming a weekend at very little cost. With the entry fee to the Colosseum and two metro tickets on Saturday and Sunday, I spent 18 euros before food.
There are many bakeries and cafes around Rome that are cheap. A giant pastry can be found for just 2 euros or sandwiches for 2.50 euros. A slice of pizza usually costs around 2.50 euros but varies because most pizzerias charge for pizza by weight. If you're looking for a sit down dinner, most pasta dishes cost under 10 euros and meat dishes cost more like 16 euros. This includes tax and tip! The prices listed at any store or restaurant in Italy are the final prices. With the current conversion rate being one U.S. dollar for 0.92 euro, prices for food in Italy is about the same as prices in the United States. Wine in Italy is much cheaper than in the America, especially at the grocery stores. In Rome, you can find a bottle of wine on sale for 1 euro, with the average price being 7 euros.
Don't forget to try gelato while in Italy! The average price is 3 euros for three scoops. Depending on the Gelaterie, you can get it in a cannoli shell cone with whipped cream on top! You won't want to go back to ice cream after trying gelato.