it is the exact same cost. in the old system, BE was worth more, exactly 6.5 times more, as riot stated basically everywhere. that is why the BE you had in the old system got multiplied by 6.5 in order to bring its value to match IP. they did that so champions that cost 6300 IP would cost 6300 BE as well.
500 x 6.5 = 3250
600 x 6.5 = 3900
the value of BE has gone down. before disenchanting a shard would get you consistently ~200-300. now its 90-1250, so the average per shard is double the average before AND you get them a lot more frequently, instead of randomly from hextech chests, you get between 1 and 4 every 10 or so games(@lvl 30+).
champ shards in the store before were 1700 IP, going by the conversion, their value now is 11 050 BE. So the fact that you can pick them up for 3950 BE means they are discounted by 65%. Sure the shards cant be rerolled into permanents so their value is down, hence why there is such a huge discount.
also for people complaing about the shards worth 90, the price is based on their cost to buy, there are:
13 champs @ 450
22 champs @1350
24 champs @3150
47 champs @4800
32 champs @6300 (33 including Zoe)
that means you have a 13/139 chance (~9%) to get a shard worth 90 BE, **HOWEVER ** you are almost 3 times as likely (~24%) to get a shard worth the max value. You have a 58% chance to get a shard worth at least 960 BE (which is why you might get 1 per lvl) and a 75% chance to get a shard worth more then 630 BE.
for all the people complaining about 90 BE shards you probably have 3 times as many shards worth 1250 and just want to complain anyway. the system is much more generous in terms of BE generation now, you just acquire it 800-1250 at a time instead of ~30-80 per game.
All this bitching based on not understanding the new system has even prompted riot to simplify the First win bonus, less experience but a flat amount of BE means you level up more slowly but receive a small amount of BE every day. Even though you probably make the exact same amount of BE in the long run, most people complaining right now will see this as an increase in the rate they get BE.