how hard is it to replace a flush valve?

I have already replaced the fill valve and flappers on a couple of 12 yr old toilets. We have a small stream of water that runs down the back of the toilet from the flush valve and the plumber is quoting $400 to fix it. That sounds ridiculous but my PHD in toilet repair only goes so far at the moment. Is this something I can reasonably consider doing myself? He also tried to get me to get a new toilet hah! no way pal... this is an old toilet and its not one of these new flush em 16 times to make something go down! Hope got all the terminologies correct.