Barcelona Zoo is situated in the Parc de la Ciutadella, near the Estacio de Franca.
Travelling in to Barcelona by train from north of the city, the best station to use would be Arc de Triomf, walk through the arch and keep going!
At €20 for adults and €12 for under 12s (under 3s go free) it can seem pricey, but you can spend a whole day just wandering round. If you take a picnic and plenty of drinks you don't have to spend another penny, or should that be cent?
There are the usual food stands dotted around the zoo, along with vending machines for drinks and ice creams. The little train that goes around the grounds, or the electric buggies, will cost you to hire, but the zoo is mostly flat, and there are areas that you cannot take vehicles so they may not be more than for novelty value.
And of course, both exits are reached through gift shops.
Being a Brit, I have to mention the toilets, and they are great, and much cleaner than a great number of UK toilets found in similar attractions.
A word of warning, please don't expect to see similar conditions for the animals that would in a UK zoo such as Chester, because it is not going to happen. Whilst the pens and enclosures are being improved (it is a continuing project that has been going on for some time now, and I can see changes each time I go) they are still very sparse and small.
The zoo is open every day, including half day on Christmas Day, from 10am, closing at various times depending on the year.
More details can be found here.