I've always wanted to try this and decided to go with my wife on my last visit. Firstly, if you are at Cagsawa you will be approached by some sellers trying to get you to go on the tours. We took one guy's number and called him later. We got a discount (200php for each of us but we had to ask). So, price was 1300php each.
We went on the Green Lava Trail and to be honest, I'm pleased we did. From what we were told, the Black Lava Trail is a bit more dangerous and adds another hour on to the trip. Green Lava Trail is about 2.5 hours but it's really fun.
When you get to the top (not the actual top of Mayon) you can take a rest for some photos and buy a fresh coconut there. It was just my wife and I plus our tour guide who took loads of photos for us. Really awesome, his name was Bong I think. Really nice guy.
Seriously, I'll be doing it again when I'm back in December. Don't miss this opportunity!