I like it!

May I?

Effects:
Prevent damage taken
Knock back enemies that are next to you.
Invincible for 0.5 second.

Cost: Scales with number of times used (starting at 1 coin) and will be displayed (somewhere) what amount it is at, up to a maximum of 25 coins.
If activated without enough coins to pay, Greed takes a random item from you as payment. (and the coins you do have)
If activated without money or items, Greed murders you. Game Over.


Realistically usable in early, mid, late game stages.
Clear Purpose. Chance to not take damage when hit (as opposed to prevent ability to be hit at all)
Clear Advantages and Weaknesses. Using too much will quickly become your death.

Maybe you could have the price scale with the floor? 5 coins for the first 6 chapters, and 15 coins from the womb onward. Or maybe have it be 1.4 coins x the chapter number, so it willbe 7 cents on Depths I and 14 cents on The Chest or Dark Room.

I'm just giving a suggestion, also, 10 coins is 2/3rds of a pedestal item from the shop, which is (in my opinion) the worst pool in the game, except for a few pretty good items.
About the shield, you could easily win The Lamb because of the duration of the shield. When you get there it is very easy to kill him, specially with this time.
Maybe, as a step further to it, you could make it an active item, and everytime you use it, it'd take 10 coins from you and give you about 5-8 sec. of shielding.
Also worth noting that you could increase the price and keep the same duration, maybe 25 cents like Greed Gullet.

well 10 coins is 2/3rds of a pedastil item so it should probably give a soul heart and a damage up when used

See this is why its hard to balance items. One guy says its too strong and shield shoudl be shorter and the other guy says that its too weak and shield is not enough... *sigh*

for 10 coins id expect more than just a little shield

Pretty cool, but you could nerf the duration of the shield to about 5 sec. or something