Ok there are two ways of getting radiant shards and that is Crota's End and grinding for eris. I have a discrepancy though, it costs 5 radiant energy for ONE SHARD, yes I said one shard. There needs to be some other way of getting radiant shards like a chance of getting them from the daily, or something other than shithead eris and RNG. It costs 7 shards per defense upgrade on the last 3 upgrades. 21 shards is quite quite hard to come by, now I know they don't want people running through everything quick but there really has to be another way of getting radiant shards.