As I mentioned in some posts before, traveling Cuba on a budget can be a bit tricky, but it is not impossible.
Havana is the best place to stay on a budget because of your many options. Here you have hostels and a lot of options to eat well but not expensive. In other areas, especially touristic towns and popular locations, you might have to look longer for such things, but they exist.
Moving around will affect your budget a lot and is yet one of the best chances to safe money. In Havana, there are a lot of possibilities to move around. The cheapest would be walking, but it is a big city and you can not walk everywhere.
A little more expensive than walking is taking the bus, or “guagua”, but really just a little. A bus ride cots 40cts from the CUP. Just be aware that if you pay with 1CUP you will not get any change back, usually and bigger bus stops there are people who change the coin to cents.
Some buses are really crowded, so always try to catch them during the first stops.
A great way to get around a little faster and still see something of the city is the P- Metro buses.
You could even safe on your city tour if you take rides on the P4 or the PC and discover less touristic areas if you take the P3.
Keep in mind that at the bus stations you should always ask for the last person in line: el Ultimo.
Have fun discovering Havana by bus.