Within 3 days we ate at Katla twice - once when staying the night in Hvolsvollur and once on the drive back to Reykjavik. Within that time they changed their menu from allowing a half buffet 3350ISK or full buffet 3990ISK to only have the full buffet option for 3690ISK. I think the move was probably because the half buffet was a much better value option for anyone who wasn't going to eat excessively.
This is how it works - there's 4 sections: soup, salad, main courses/hot food, dessert. Half used to be 2 courses (main + other) and full is all 4. You get to fill up your plate and go back for one serve of seconds. It's a lot of food and noone really is going to eat 8 plates of food. 4 on the other hand is good value and you can even share bits (naughtily). So they removed it and such is life.
The soup options are a salty lamb with lots of vegetables, a creamy seafood which was a little scarse in seafood, and minestrone I didn't try. The salad options were mainly leaves and not overly exciting. The mains covered Portugeuse salted cod, Greek style ling, Chinese chicken, pork ribs, glazed chicken, rice, meatballs, some steamed vegetables, some kind of fajita and best of all roasted pork and the even better and excellently tender roast lamb. The lamb was easily the best dish with mild flavour, pink middle and very tender. Unfortunately for all of us they didn't have it available on the 2nd trip.
What's also good is that guides eat for free, which certainly factors into costs when they are this high.
If I was there early enough, looking around the lava/volcano centre in the attached building and the shop with many skins would be an extra.