Improved Invis.
As has been pointed out a billion times, the item creation rules are only rough guildines. They are not meant to be hard and fast rules, and all items thinkable by using them are not automatically "part of the core rules".
For simple items, or very similar items to existing ones, they provide a convenient way to DM's to quickly come up with a price that is balanced through theoretically previous playtesting. This applies nicely to the skill bonus, abilitiy bonus, and non-abusive spell items (I'm just talking wonderous items here, since others are bit more clear cut).
Now, as it even says in the core rules (so is "part of the core rules") and has been repeated by the creators, some items have been adjusted because they far more powerful than the pricing would suggest by following the equations (or, rarely, less powerful). There is no quick and easy formulae for adjusting the price and availability of these items, and it is left up to the DM's judgment and common sense to figure out something appropriate for his or her campaign.
A useful guidline suggested by many, including the creators, is to compare to existing items that seem of roughly equivalent power level. This is usually better than coming up some sort of multiple of the calculated cost, though even this can be difficult too with some weird items that are hard to compare.
OK, enough with the generlized lecture. As regards a ring of improved invisibility, I think the easiest way to price this item would be to base it on the ring of invisiblity and up the price by a sufficient "power boost" cost.
One good way to start the analysis would be to look at the two spells themselves and compare them. Besides being 2 levels higher (or double the level depending on your perspective) it has a significantly shorter duration. This is pretty big difference, and although you can argue that the improved invis is still fairly long lasted at a minimum of 7 minutes, it will typically only be one combat per casting (we only need to look at combat here, since that is the only important difference between invisibility and improoved invis.). So, a 4th level spell that only lasts one combat, though all attacks in that combat, vs. a 2nd level spell that lasts potentially hours, or just until the first strike. To me this suggests at least a doubling in price from invisiblity, but that's just a first analysis.
You could also look at who would benefit most from this ring, and what level you expect the characters to get it. Once you start getting around 12th level it might not be such a big deal since invisibility purge, dispel magic, blindsight, scent, true seeing, etc. are starting to be much more common with foes, and characters at that level often have access to becoming Ethereal with can be much more of a problem is some situations.
Rogues are the most obvious for benefiting the most from this. Now they and a wizard could both use a wand of improved invisibilty to get a similar effect. So lets look at a wand ... 50 charges for around 21000. Wand of invisiblity ... 50 charges for around 4500. Now that's a really big difference in price, and would suggest a bigger price jump to me than just doubling.
Cloak of etherealness, both better and worse than improved invisibilty depending on your goal (immune to attacks, but can't attack) is usable 10 minutes a day and costs 52,000 gp. Ethereal Juant is a 5th level Cleric, 7th level wizard spell, with a shorter duration than improved invisiblity. This seems pretty good item to compare to I would think.
My end opinion after all this blathering? I think I would allow a 10 minutes per day ring of improved invisibilty fo 50,000, unlimited closer to 70,000 - although this could go up or down, 10 minutes a day is often unlimited, just stops you from going around all the time invisible, but that could cause rollplaying problems anyway if handled right (your party can't see you either).
Ok, there's a long winded, potentially useless, analyses of the whole situation. Just though I'd add my 2000000 cents.