Cartagena is full of wonderful buses, some are very old and dangerous and some have televisions and air con. Most bus journeys cost 1200 pesos around 50 cent in euros. The air con buses cost 1500 pesos and are probabley much better if you want cold air. However most buses stop for everyone and if you dont get a seat it can be a very uncomfortable ride, crowded with people and some of the older buses are quite dangerous, breaking down or windows that you think will fall on top of you at any moment. Although it is an experience to take a trip in one of the many on the road. Just put your hand out, if you do catch one the conductor normally takes your money, try to have the exact change, otherwise you pay the driver if the bus does not have one as soon as you get on. Expect salsa music blaring out and holy objects hanging from the bus, ripped seats and in some cases vendors getting on the bus to sell you water, etc, or tripping over boxes of fruit as the vendors go into town to sell their wares. All in all its a great experience and l loved travelling by bus, much more exciting than a taxi and cheaper too.
Taxi's are cheap and always available anywhere anytime. You should have a prepaid cellphone while there so you can call for cabs if you want an A/C taxi. Always check your belongings- make sure you dont leave them in a cab or you'll never see it again. Dont pay more than 5000 peso unless you are going far. Always ask how much it will cost before you start.