I know that a free spell is powerful. I know that Wizards wants weaker spells. By why are 9 out of 11 cipher spells 4 cmc or above, with a large number at 4? Why couldn't Last Thoughts cost ?

Sorry for this, guys, but I need someplace to rant. Let's compare Last Thoughts to Curiosity.
Pros of Last Thoughts:
-Can play without a creature
-Draw a card when you play it
-Harder to remove without removing the creature
-Triggers Extort
Pros of Curiosity:
-Interacts with things like Sphere of Safety
-Works very well on Prodigal Pyromancer
-Can potentially be recurred

What if Last Thoughts was an enchantment?
Last Thoughts
Enchantment- Aura
Enchant creature
When ~ ETBs, draw a card. Whenever enchanted creature deals combat damage to a player, draw a card.

Is this appropriately costed? I don't know. To me, it seems too expensive. Certainly most Cipher cards are too expensive to see constructed play. Clearly development was afraid of their limited implications.

I think part of the reason why the cipher costs look expensive is because Wizards is assuming you're going to get two activations out of it immediately (since blue/black is excellent for evasion), and you get an enchant creature that can't be disenchanted in the trade. So for Last Thought, you wait for your opponent to tap out (easy in the early game)pay , swing with your 1/1 flyer, draw two cards and get an unremovable Curiosity in the exchange. That seems... pretty damn good, if you ask me. The stars, of course, need to align... that's why Last Thoughts can cost the same as Inspiration after all.

If you play enough modern, commander or casual, I can see why Cipher would look terrible. Creature kill is very common in those formats. Last Thoughts will just be an over-costed wash. In limited and maybe standard, the removal just isn't there. Not in those sorts of numbers (and possibly at sorcery speed.) A single cipher spell can potentially trigger four times without much interference if the opponent only wishes to race.

Last Thoughts is 4 mana, not 5, so I'm not sure why your Aura version is . I think your point is fair enough without needing to overstate it ;)

Yeah, I don't see why it couldn't be . I guess blue was seeming too good in Limited at that point (although Shadow Slice seems very expensive too). I do note that there's a lot of cipher enablers - blue has two commons saying "unblockable" and one uncommon, and there's intimidate and "unblockable-except-by-Rogues" at common too. So maybe having good cipher enablers meant that the cipher cards themselves had to be upcosted?

Ok, so if I understand it; this is, pretty much, strictly worse than "Enchanted creature has +1/+0" (multiplayer shenanigans aside)

I think the irritation of undisenchantable enchantments is a good reason to keep them costly. (Add "Shroud" to your example, is that justifying the extra cost?)

That is:
Last Thoughts
Enchantment- Aura
Enchant creature
Shroud. When ~ ETBs, draw a card.
Whenever enchanted creature deals combat damage to a player, draw a card.

I was just rambling because I was so disappointed in Cipher. I wish things like Cipher and Assemble the Legion didn't have to be costed so high primarily because of limited. It makes me even angrier because silver bullet hosers that shouldn't be printed, like Grafdigger's Cage, Illness in the Ranks, and Homeward Path get printed at ridiculously low mana costs.