Public transport in La Paz is definitely not like what you might be used to, no matter where you come from. It is not as bad as it might look like though, in spite of the lack of set times and schedule - but still, it is not easy to understand and most of the tourists would rather pay a bit more for a taxi ride rather than boarding a minibus and risk getting lost in some unknown dark part of the city. That is perfectly understandable and will definitely save you a lot of time.
Still, there are a couple of tips you should know in order to avoid disappointment.
Basically, there are two kinds of taxi in La Paz: taxis and radio-taxis. Or, put another way, there are taxis with nothing but a sticker saying "taxi" and taxis with a proper "bubble" sign on top, that suggests* they belong to a real official registered taxi company, that can be tracked and are always connected to a central radio station that sends them all over the city. If it weren't clear enough: please go for the latter only.
* We asterisked this because there are some taxis that have a bubble sign and seemingly look like they are part of a company but they are simply a "one man company" . They will have no radio (other than one to listen to the football). You can recognize these as the phone number will just be a cel phone number often starting with a 6 or 7, rather than a landline number starting with the city code number such as 2 in La Paz or 3 in Santa Cruz.
Radio-taxis are more reliable and if you ask your hotel to call a taxi for you, they will always call one of them - they might also even have a specific driver at their service. I mean, sticker taxis are not all bad and most of them will definitely take you to destination safe and sound (especially those with a black and yellow checker on the side - they are registered ones), but being foreigners it is recommended to avoid them. Unfortunately there have been people getting in taxis and being setup for a robbery by waiting thieves on the route the driver takes you to especially at night.
It is always hard to gauge how much you should pay when you're in a new city or country so here you have some approximate prices for the most common taxi rides in La Paz (La Paz is the most expensive city for taxis in Bolivia so you should expect to pay a little less in other places).
- Getting to the main bus terminal shouldn't cost you more than 15 Bs from the center of the city (near San Francisco church / Prado areas) and 20 Bs from Sopocachi - please consider some extra 10 Bs during night-time.
- Getting to/from the airport from the center is about 70 Bs - up to 90 if the sun is not up yet or is late at night.
- From the center to Irpavi and the Megacenter Cinema Complex should be about 25 / 30 Bob (again late at night a little more)
- From the center to Mallasa for the Valley of the Moon should be 35/40 Bolivianos although they will commonly try to charge you more as many drivers (and people in La Paz in general) seem to think this is alot further than it actually is. A taxi back from Mallasa is cheaper as the drivers in Mallasa themselves know the prices and distance better so it is often 30/35 bob. These prices might vary though, it really depends on how "gringo" or rich you look! So please polish your Spanish and try your best! Also please note that it is regular practice to increase the price a bit if there are more of you and/or you have lots of heavy luggage.
Oh, I almost forgot: always look at the taxi driver in the eyes before boarding - if you think he would be a perfect actor for the next movie about the life and adventures of Pablo Escobar, you might reconsider catching a minibus - it was not so complicated after all.