For a magic item that gives another 0 lvl slot, i'd use the Pearl of Power to compare price.
When multiplying spell levels to determine value, 0-level spells should be treated as 1/2 level.
So to grant an extra 0 lvl spell slot, should cost 500gp.
But this provides more of a bonus than if you got one 1st lvl slot that expires when cast. Maybe 750gp, me thinks (this is RAI).
What's the 2nd item meant to do? What do you mean 'only one set'?
Actually that would probably work better.
If you make it 500gp and gives one extra cantrip able to be memorised per day, that should be balanced.
RAW there is already an item that allows you to use an extra set cantrip an unlimited number of times per day: Hand of the Mage.
http://www.d20pfsrd.com/magic-items/wondrous-items#TOC-Hand-of-the-Mage
Aura faint transmutation; CL 2nd
Slot neck; Price 900 gp; Weight 2 lbs.
DESCRIPTION
This mummified elf hand hangs by a golden chain around a character's neck (taking up space as a magic necklace would). It allows the wearer to utilize the spell mage hand at will.
CONSTRUCTION
Requirements Craft Wondrous Item, mage hand; Cost 450 gp
Its 900 gp and takes up an item slot. On the other hand, it also doesn't require you to be able to cast the spell normally. If you only want spellcasters to get some use out of the item, cheaper is probably ok. If you want anyone to use the cantrip, but it not take up a body slot, then it becomes more expensive.
A Hand of the Mage that requires you to have Mage Hand on your spell list should be worth 450 gp, based on the ratio of potion and scroll prices. (A potion costs twice as much as a scroll, but does not require the spell on your spell list.)
Making it slotless would put you back at 900 gp.
To allow the user to prepare an extra cantrip, I would increase the cost by 50-100%. There are not a huge number of cantrips, so it is probably often going to be used to prepare the same cantrip and rarely one or two others.