Well, you left out some details about the item, but I'll take a crack at it.
First you have to figure out the "general formula" for this item. That's my term - not in the books - but its based upon the chart in the DMG for creating magic items. For example, let's use the simplest formula for a command-word activated, 1/day item (this assumes the item is in the "correct slot" and its not being added with any other powers.):
spell_level x caster_level x 360gp
Once you determine the "general formula" for the item, then you have to fill in the blanks - spell_level and caster_level. Check out these cases:
1) Let's say I am a 19th level cleric PC making this item. Its an 8th level spell for me, so that's easy. The minimum caster level for me to cast this spell is 15th level, so I can choose a caster level anywhere between 15th and 19th for the ring, and the price adjusts accordingly. The effect from the ring is exactly how you made it - you get what you pay for. For example, if I choose 15th level, then the total is 43,200 (8 x 15 x 360). To make the item, I pay 1728 XP and 21,600 gold.
2) Let's say I am a 19th level druid. Its a 7th level spell for me, and my minimum is 13th level, so I can choose anywhere from 13th to 19th for my caster level. At 13th level effect, the total is 32,760 gold. To make the item, I pay 1310 XP and 16,380 gold.
3) Let's say I am a 19th level sorcerer. Its a 7th level spell for me, too, but my minimum caster level for the spell is 14th, so I can choose between 14th and 19th for my caster level. At 14th level effect, the total comes to 35,280 gold. That means 1411 XP and 17,640 gold.
4) Wizard - same as druid, #2, above.
Now, after the item is made, let's assume the player wants to sell it. (This probably wouldn't happen much with such a high level item, but lets see...)
As the DM, you decide that the King's Royal Scout (a Ranger) will buy the item for 75% of market price, because the party has completed an important mission for him. So, what's the market price? Per the SRD, the market price is always determined by the cheapest way to make the item - so, you would always use the Druid's price, since he can make the item the cheapest. But wait! You still have some math to do, because of the caster_level of the item.
The cleric's 15th level item, made by a druid, would have a market price of 37,800.
The sorceror's 14th level item, made by a druid, would have a market price of 35,280.
So, casters which cast a spell at higher level than normal (the cleric in this example) usually "lose" a little more than casters who cast it at lower levels.
Most of the trouble comes from trying to determine the "general formula", once you decide on that, applying the concepts of spell_level and caster_level is easy. (For a recent example, search for the thread on "Boots of Expeditious Retreat". You can't use the formulas from the DMG for this, because the item becomes too underpriced for its power.)