Bullfight tickets vary in price according to their position in the bullfighting arena. The ones in the shade (sombra) are more expensive than the ones in the sun (sol). Some seats are designated as sun and shade (sol y sombra) because they begin in the sun but move to shade as the afternoon progresses. Seats located closest to the bullfight are also more expensive than those near the back. So ones near the ringside barrier (barrera) which are also in the shade are the most expensive.
Buying tickets at the actual arena can prove a major hassle as it is difficult to work out which ticket window (taquilla) you should go to and without a very good level of Spanish it is almost impossible to explain or find out which tickets you wish to buy.
Just study the poster of the event in advance to decide where you want to sit then ask for those seats or simply copy the name of the seating area and show it to the ticket seller.