The transmission is a complicated assembly and many different things may go wrong with it.
Some easy-to-reach small parts may cost as little as $20 including labour. At the other extreme, it may cost hundreds of dollars to repair or rebuild a severely damaged transmission.
Your best bet is to invest in inexpensive routine maintenance and get your transmission checked by a competent shop such as National Transmissions. One thing is for certain, your transmission will not get better on its own!