They should cost a lot. I'm glad they're rare.
Hope it involves new ways to use diamonds up. They certainly aren't going anywhere else these days.

Diamonds are more or less useless in trades (as told by several ppl above me), the best way to get a rare item imo is to pay for them in wanted items, with wanted items I mean items such as: ec's, brutals etc. By gathering a huge amount of these wanted items the chances of you reaching your goal item will rise immensely compared to when you will pay with huge amount of diamonds.

as for "I feel that any item obtained through Event Coins you can trade back for the same amount of Event Coins that you paid." is just a stupid idea.

people would trade their old event items in for the same amount as they bought them, meaning that their would be a huge amount of ec's going in circulation. This would create a terrible rise in the prices of the event items, making it harder for new players to gain a event item. Instead EC's should be used up as much as possible. By doing this it will motivate the players to play more events.