I think Croatia has rapidly woken up to being a tax free zone outside the EU, and has lifted its prices to make them similar to the EU - so while they are not yet in the EU (and avgas remains tax free) the outlet makes huge profits - perhaps 75% of the selling price. These people aren't stupid...
I paid 50p/litre at Losinj LDLO in 2007. One DA40 (not sure of avgas or diesel) owner I got talking to down there said it cost him just £20 to fly back to Austria
Around the EU, avgas prices are pretty similar - I'd say within a 20% band generally.