I had to have the memory battery replaced two months ago. I opted to have the terrain module and database installed at the same time the box was opened up at the local avionics shop. Total bill about 600€, inclusive the battery and terrain update.
I tested the terrain module by flying straight towards a TV tower and about 1NM from impact it clearly showed the warning on the screen overriding the current display. Well worth the price even if you never intend to use it.