When to get crit item instead of damage?

All of the other items you mentioned are bought largely for their extra features, and the damage is only a bonus

People get Sange because they need more HP and to avoid being kited. People get Yasha for the move speed or to build into manta for the illusions. Battle Fury is a farming and split push item which lets the hero get more items quickly.

Crystalys is normally a 3rd-4th item, after the hero "fixed" all of the limitations of the hero with other items

Lets look at Sniper. He has some pretty damn good attack speed, a passive bash, and incredible range. He would LOVE to just sit a mile away from the fight and crit people til they are dead. He can't just go straight for crit though. First he needs to get some survivability (normally Shadow Blade or Mask of Madness). Crit doesn't work against towers, so he might want a Manta Style too. If the team lacks any AOE, he might even want a Maelstrom before crit. Daedalus doesn't give any attack speed, so does he really want it over a Butterfly when he could bash lord people from a mile away?

Your math is actually off. Crystalis crit contribution is more like an overall 15% damage boost. (20% chance to do an extra 75% damage on a single attack, is an average of 15% damage boost per attack in the long run)

Icefrog has balanced most of the items to be roughly of the same cost-effectiveness per damage done, so you will be picking the item based on the utility it provides you. Daedalus eventually gives you the highest overall damage but it also gives zero utility.

Some comparison. Say a level 16 Mirana is used as a baseline (no talents or steroids) taking all skills, one stat point, ignoring the 16% atkspd increase buff for now, she has average damage of 87 and 0.96 attacks per second. Assume her core items include Phase Boots, Drums, Ring of Aquila, which give her now 138 damage and 1.07 attacks per second. She can choose her first DPS item. Yasha gives a 31% damage increase, Crystalis gives a 40% damage increase, Maelstrom gives a 58% damage increase. Once normalized for weapon costs, Maelstrom is the most efficient, followed by Crystalis, then Yasha.

Looking at more expensive items

You could look at Daedalus (114% increase) versus Mjollnir (111% increase) versus Desolator (85% increase) and MKB (Monkey King Bar) (105% increase) which, once your normalize for cost, all have the same cost efficiency, except for Monkey King Bar which is slightly worse than the rest. In which case, the choice is purely down to functionality - Daedalus gives the best scaling damage in super late game (crit interacts with other damage items, but that is a long way away - this only applies when you're 6 slotted) but doesn't work against buildings, while Desolator occupies an orb slot and is excellent for sieging buildings while having great synergy with other -armor items on your team, and Mjollnir is the only ranged weapon with AOE, and has interesting utility by being able to pop off an opponent's Linken's charge, and the lightning shield can do a surprising amount of damage as well. Monkey King Bar is slightly less cost efficient but has the most utility, preventing miss chance (which is huge) and also producing mini stuns.