The beacon value ranges a lot. It can change depending on how quickly you are wanting to sell/buy the item. I would sell beacons for 200-230 and personally not buy one for anymore than 210. If mob arena returns soon, they will likely almost half in value again. If you are in no rush to sell the beacon, you could probably get up to 250 grass for it. Netherstars on the other hand go for significantly more (maybe even up to 300grass). This may sound surprising bearing in mind a netherstar is needed to craft a beacon, although this is likely due to the fact that most people craft their netherstars into beacons, therefore the netherstar demand increases, therefore increasing their value. That's a bit expensive.
