The exchange should not have a ratio. That would be pretty lame. It should be a simple 1:1 shard conversion. The available characters on the exchange, per the post, will be alternated with the exception of only one (the 'community requested' one). Therefore, the value proposition implies a simple 1:1 exchange of shards which is actually exactly how it should be given that it would be the same as assigning a character to any one of the other shipment locations. The farm is the same, the time gate is much longer (due to needing to be 7* to save up shards to be able to shop with). The TIME is the challenge and true cost of the toons available on the shard exchange. If it is not 1:1, that would be a complete and utter travesty and _extremely_ poor development with a very strong eye towards a unethical money grab.
