Generally, items have only one caster level, and all powers on the item work at that caster level. With the caster level 25 power, that would increase the cost of the rest of the power.
Constant divine favor would cost a little or a lot,depending on how rules lawyery the creator is. The cheapest version would use:
1 * 25 * 2000 * 4 (duration) * 1.5 (secondary property) = 300,000gp
Another way to look at it would be:
6 * 6 * 2500 * 10 (epic bonus squared in a second property) = 900,000gp for the plus to hit. and another 900,000gp for the plus to damage, for a total of 1,800,000gp for constant divine favor.
You could also mke it persistent divine favor castable once per day. That would drop the cost to:
5 * 25 * 1800/5 * 1.5 = 67,500gp
It would also leave it more vulnerable to dispelling.
Note that the first option is correct per the constant spell rules, whereas the second version is correct per the "item provides a bonus" rules. The bonus version of the rules is what is actually used in the DMG, otherwise bracers of armor +4 would cost 2,000gp for mage armor constant.
