I want a weapon with an activation rune, which I can thumb to boost it's power. Sort of a WH or WH40K feel...
The weapon must be at least a +1 standard magic weapon. By activating the rune, whatever additional properties that are contained within the weapon manifest for up to 10 rounds per day (need not be used consecutively).
So, a +1 longsword can become a +1 flaming longsword for up to 1 min. When the additional powers are added, the base price and cost are adjusted to 37.5% of their usual total (effectively modifying the x2000 multiplier to x750).
This does have the effect of putting more powerful magic items in the hands of lower level characters, since they will be able to afford them earlier.
Is this just too much? Is there something I am missing in even considering this?
Well,
If you look up the rules on magic item creation, you find the rules for 'charges per day' to be Price/(5/charges). That's not a perfect solution, but it's a rule of thumb. It's really designed for spell effects. So, we could have a spell that lasts for N rounds (Where N = Caster Level) and activated it X times per day and we'd know how much it costs [(Spell Level * Caster Level * 2000)/(5/X)]. We could take a similar approach with the attribute.+1 to +2 = 6,000gp
10 rounds per day is honestly a bit like one spell at CL 10 that lasts 1 round per CL. But it's got more utility, since you can turn it off to conserve rounds. So... let's treat it like 3 charges.
6000gp/(5/3) = 6000gp * 3/5 = 3600gp
So, say adding that ability to a +1 weapon would make it a 5600gp item. I don't think it's too over powered. That doesn't include the weapon's base cost or masterwork of course.
Personally, in my games, I looked at the Magic Item compendium and saw the weapon crystals that duplicate the flaming/shocking/etc +1 attributes being sold for 3000gp, and just made all flaming/shocking/etc enhancements cost 3000gp. Doing that, you'd have 3000gp * 3/5 = 1800gp, or 3800gp for the weapon (plus weapon costs and masterwork of course).
Just my $0.02