According to Paul McCabe of Boston College
As mentioned earlier, gladiators were trained at special schools originally owned by private citizens, but later taken over by the imperial state to prevent the build up of a private army. Gladiators trained like true athletes, much like professional athletes do today. They received medical attention and three meals a day.
Understand that the gladiators received training, by soldiers, other gladiators, etc, and this took time. Time is money, in that all the while the gladiators were being trained they were being housed and fed, and a gladiator, even if slave, could not "work the field" or anything else a slave might do, because their lives were taken up by the training needed to allow them to "fake combat" realistically. I believe that gladiators were the WWE Entertainment wrestlers of their day. While their matches inside the amphitheaters were scripted to an extent, they still needed to be able to make the crowd believe the combat, and two men (and women, believe it or not) swinging implements of destruction are going to cause wounds, large and small.
Their training included learning how to use various weapons, including the war chain, net, trident, dagger, and lasso. Each gladiator was allowed to fight in the armor and with the weapons that best suited him.
They weren't trained how to just stand there and look good, but were given training in multiple weapons and armor. While each was allowed to fight in the armor of choice, if today's exhibition required one 'side' of the match to use daggers and tridents, with the other using swords and spears, then that's what they used. If you had never even picked up a spear before, you'd fail miserably in the entertainment, be booed, and perhaps get the 'thumbs down'.
They wore armor, though not Roman military armor as this would send the wrong political signal to the populous. Instead gladiators wore the armor and used the weaponry of non-Roman people, playing the role of Rome's enemies.
Later on, gladiators were not only slaves, but ex-soldiers and free men as well. An ex-soldier would be intimately familiar with the (Roman) arms and armor he had worn during his service, but the gladiators rarely appeared as Roman troops, in the current garb of the day, so even the soldiers, those with the most training already, might need other training on how to properly use weapons and armor used by the barbarians, or perhaps Roman legions of by-gone eras. (The whole gladiatorial period lasted several centuries, and times change.)
For instance, a gladiator might dress as a Samnite in Samnite garb that included a large oblong shield (scutum), a metal or boiled leather grieve (ocrea) on the left leg, a visored helmet (galea) with a large crest and plume, and a sword (gladius).
The gladiators lived in barracks built especially for them, which were usually located near their home amphitheater. Because they were such expensive investments, gladiators were well fed and received the very best medical care of the day.
Much as the entertainment wrestlers of today, wounds and injuries were common. Just because the gladiator you face today is a colleague, perhaps even friend, doesn't mean that gladius he (or she) is swinging won't slice open your skin if you stumble the wrong way. So the gladiators were provided the best of medical care, at least as was known then. Plus, while pure slaves might have to live on scraps and be lucky to count one good meal a day, the physical exertions of gladiators meant they had to be fed not only with more regularity, but better quality as well.
Also, a gladiator usually did not fight on more than two or three matches each year. The gladiators from their certain ludi traveled together as a group, known as a familia, along with their lanista (trainer), from town to town throughout the Empire for gladiatorial games.
This was well shown in several movies, including Gladiator (2000). A familia would travel the countryside performing for the towns. This meant that there would need to be (in addition to the gladiators and the lanista) support staff as well, the owner, his slaves, perhaps his family, plus people to buy food and procure housing, etc.
All of this added up to the best gladiators costing a minor fortune, and even the not so great ones being more expensive than a normal person, slave or not, to care for, train, feed, and tend to.