There is no set formula to calculate what you want. However, by looking at the apparent formula used to generate the rings you list, we can approximate a good amount.
The formula used to get these values appears to be:
Resist Energy Base Spell = Spell Level 2
Caster Level for Res 10 is 3, Res 20 is 7, Res 30 is 11.
Base Spell Level X Caster Level X 2000 = cost.
Now, if the minimum efficiency for the 2nd level spell would be 10 resistance, then a spell granting resistance 5 should probably be 1st level. As such, it should be available at 2st level.
Using the above formula, we get a total cost of (1 X 1 X 2000) 2000 Gold.
Is that appropriate? The only way to tell is to compare it to other 2,000 gold rings.
The first one is protection +1. Would a low level PC want to trade a +1 ring of protection for a resistance 5 ring? Maybe ... Some would, some would not. As such, that is a pretty good indication that a price of 2,000 gold is probably pretty fair.
I'd go with 2,000 as the price.