Mystic Remora 
Enchantment
Cumulative upkeep {1} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
Whenever an opponent casts a noncreature spell, you may draw a card unless that player pays {4}.
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Cards that do something similar expected a card that isn't here?
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Rhystic Study 92%
$70.26#44
"Whenever an opponent casts a spell, you may draw a card unless that player pays {1}."
Esper Sentinel 89%
$56.22#76
"Whenever an opponent casts their first noncreature spell each turn, draw a card unless that player pays {X}, where X is this creature's power."
Hermit of the Natterknolls // Lone Wolf of the Natterknolls 86%
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"Whenever an opponent casts a spell during your turn, draw a card."
Mangara, the Diplomat 86%
$0.25#616
"Whenever an opponent casts their second spell each turn, draw a card."
Kraum, Ludevic's Opus 86%
#6811
"Whenever an opponent casts their second spell each turn, draw a card."
Ghostly Pilferer 84%
$3.49#1404
"Whenever an opponent casts a spell from anywhere other than their hand, draw a card."
Wavebreak Hippocamp 80%
$0.83#2329
"Whenever you cast your first spell during each opponent's turn, draw a card."