I had to have the memory battery in my Garmin 430 replaced a couple of years ago. The battery itself could be found on line for about £20. Garmin quoted me a completely ridiculous price. I went to my local friendly avionics man who told me that he had never done one before but he was game to try. Apparently Garmins put the battery in the most inaccessible place possible so that the unit has to be taken apart to get at the battery.
Anyway, the job was done for about £150.
The only drawback is that you will lose everything in your memory file and have to start again building up your database.