For gauging the power of magic items, I tend to rely heavily on the excellent Sane Magical Item Prices document that tends to be pretty well researched and highly regarded. Thus far, I’ve always agreed with them on their prices and have taken to allowing those prices without consideration.
That is, until for my game last week I needed a simple, not super powerful defensive item and figured the Sentinel Shield would be a nice little reward, only to see later that it’s listed as being worth 20.000 gp.
I’m really stumped at why it would be listed as being this powerful. According to the document, the Sentinel Shield is on the same level of value as a +3 weapon or armor, the Armor of Invulnerability, a Ring of Spell Storing or a Mirror of Life Trapping.
Is getting Advantage on Initiative and/or Perception really that amazing? Or am I missing something? Or is it just grossly miss-valued?
To answer your question of just how awesome that is, my personal answer would be pretty awesome. You get advantage on all initiative checks, which is incredibly handy, and you have advantage on all Wisdom checks that involve perception. For high-level characters, this may not seem like much, but give that to a level 10 or higher character and that is an epic boon. They get a better chance to strike first, or a better chance to escape with their lives, or hide, or even find more loot via hidden doors, as those require a Perception (Wisdom) check. The Sentinel Shield is a useful item for all characters of all levels.
The reason for the Insanely high price of the Sentinel Shield from the Sane Magical Item Prices document may simply be because the shield grants powerful benefits comparable to a feat, like Alert, in addition to not requiring attunement. Another reason why it might be so high is there is a minuscule chance of getting one through random rolling. So, rarity and usefulness do play in a factor. However, it apparently is not factored into the XGtE formula.
If you roll for random loot, you are most likely to get it from challenge levels 5-10 from a treasure hoard. That looks to mean like about a mid-level item, but it is possible to obtain it from a 1-4 challenge level hoard. You can obtain it from a roll of 86-97 on 1-4, and 81-94 from 5-10. Then you need to roll a 19-21, so the odds are slim. It does grant advantage to Initiative and wisdom checks, so that is valuable all throughout your character’s life, and its recommended price from the buying guide in Xanathar’s Guide to Everything is actually much cheaper then what is listed in the Sane Magical Item’s prices. XGtE lists it as 1d6 x 100 GP. This is on page 126 of XGtE, and the formula comes from it being an uncommon item.