Glacial Shroud is already one of the most amazing buys in the game - for 425g, you get 225 mana (note that 200 is what a 400g blue crystal gives you) meaning you're already paying under 20g for 15% CDR. This is before frozen heart, which is 630 for another 5% CDR, 75 mana, and 19 armor (Mana would cost ~150g, the armor costs ~300g, CDR is as always a hard cost to codify, but it makes it the second highest CDR item in the game along with morello's), before taking into account the 1000 range -20% dps-from-ad-carries aura.
Compare thornmail, which is 2000g for 100 armor and a 30% reflect passive (the armor itself would cost ~1500g, you're paying 500g for a passive, making it the most expensive passive in the game, discounting tiamat-minus-the-regen-stats) and is absurdly situational (Does the ad carry attack you? Is nobody else attacking you so the reflect doesn't get dispersed among their team? Am I able to force them to attack me despite my armor?) as well as Last Whisper cutting the armor down to 60ish, while FH still remains just as powerful with the aura.
t;dr frozen heart is the most cost-efficient physical defense item in the game if you have mana and don't hold a heart of gold for 30 minutes
i can go into a rant of frozen heart vs randuin's but i bet that'd bore people even more