This answer only really pertains to 3.5 and Pathfinder. I don't know enough about 4e magic items to provide useful feedback on those.
Stasis Bag: This depends a lot on the limits of 'almost any size'. The biggest bag of holding holds 1500 lbs and 250 cubic feet for 10000 gp. If that's big enough for you, then I'd say that the following would probably be fair:
Bag of Stasis
Aura strong conjuration; CL 15th
Slot -; Price 25000gp; Weight 1lb
The Bag of Stasis works like a bag of holding IV, with the following exceptions:
The Bag of Stasis only weighs 1 pound.
Any object may be placed in the bag, as long as at least some part of the object can
fit through the opening, which is 6 inches wide.
Any objects place into the Bag are put in stasis. No time passes for objects
placed in the bag.
Creatures may not be placed in the bag.
Craft Woundrous Item, secret chest, temporal stasis; Cost 12500 gp
Secret Chest, Temporal Stasis.
The price is based on the standard price of a Type IV bag of holding (10000 gp). I used the price for making a slotted item unslotted, which doubles the cost, because the Bag IV weighs 60 lbs, and bringing that down to 1 makes it way easier to carry. I then added an extra 5000, which is just eyeballing the cost for the stasis effect. Since you can't put creatures in the bag, I don't think that warrants the full cost of the Temporal Stasis effect.
Life Savers: This one is much easier.
Aura strong conjuration; CL 15th
Slot -; 1350 gp; weight negligible
The Life Saver is a small pill that resembles a piece of candy. When eaten as a full-round
action, it provides protection against death temporarily. If the recipient dies within the
next 5 minutes, he heals 5d8+15 points of damage. If this would bring his hit point total
above 0, he comes back from the dead. If the recipient comes back from the dead in this
way, he gains 1 negative level that lasts for 1 day.
If the recipient attempts to use a second Life Saver within 5 minutes of taking the first,
he dies immediately. This counts as a death effect for the purposes of resurrection.
Craft Woundrous Item, Breath of Life; Cost 675 gp
Breath of Life
The cost of this was determined as follows: Contingency as a single use, use-activated item at caster level 15 costs 6*15*50=4500. The caster level is 15 because in order to have a contingent Breath of Life, you need to have a caster level of 15. A single-use, use-activated Breath of Life would cost 5*9*50=2250. Total, they cost 6750. However, contingency lasts 1 day per level, and this effect is only 5 minutes, and can't be used twice in one battle, so it's much less useful. Because of that, I divided the cost by 5. Since a scroll of Contingency at level 15 plus a scroll of Breath of Life costs more than this item and is significantly more useful, I think the price is fair.
Edit: By request, the Greater Life Saver.
Life Saver, Greater
Aura strong conjuration; CL 20th
Slot -; Price 10425; weight neg
The Greater Life Saver works just like the Life Saver, but a slain user instead is
resurrected, as the spell Resurrection. This will raise the user in much more varied
circumstances. Disintegration and death effects are no obstacle. The user is raised
one round after his death on his normal initiative count, with full hit points.
The user gains a negative level for one day after being raised.
Craft Woundrous Item, Resurrection; 5203 gp
The pricing here is the same as above, but the cost is for a level 21 caster to cast contingency, and level 13 caster casting resurrection. Resurrection also has a 10000 gp material component. The total cost is: (6*21*50+7*13*50+10000)/2. I divide it by 2 instead of 5 for 2 reasons: This item is very much more powerful than the basic version, and even if you don't need it to save your life, you can kill yourself at the end of the fight before the duration wears off to completely heal yourself. Note that this doesn't actually require a 21st level caster to cast contingency, that's only a shorthand for pricing.