Drow Dagger of darkness: 1d4 + 1 damage, additional +1 when fighting lawful-good creatures
Elvin knife of light: 1d4 +1 damage, additional +1 when fighting chaotic evil creatures
I honestly don’t see how either of these could be overpowered. They are simple +1 weapons; so far all my characters have one (with the exception of a spiked chain form of a weapon, which is 2d4+2) with a non-specific magical effect that deals additional +1 damage against one specific alignment.
Both of these weapons are ancestral weapons for the characters, passed down from generation to generation. They are also the only magical weapons in the village. Therefore, they would have no need to pay for them.
Technically they're not.
+1 dagger (2,302gp) grants a +1 enhancement bonus to attacks and damage. While your extra +1 to damage (conditional to a specific alignment) is nowhere near the extra +1 enhancement to attack and damage a
+2 dagger (8,302gp) would cost, it's not 0gp worth of extra value.
That being said, there's an answer.
WotC released the Magic Item Compendium a couple years ago. They introduced "weapon crystals". You can snap one (and only one) on a weapon at a time. You can keep a few of them in a pocket, but only one can be clipped on. A
lesser energy assault crystal (fire) just happens to add 1 point of fire damage to the weapon's output. Next, the same without "lesser" involved just adds the usual +1d6 fire damage. The lesser costs 600gp and the normal costs 3,000gp. Since
flaming is normally a +1 price, it would normally add a minimum of 6,000gp to the price of a weapon. Okay. Fine. So the equivalent of the "lesser" crystal theoretically would add 1,200gp as a permanent enchantment on a weapon. Finally, let's come back to the idea that you've got an alignment restriction in place. PFRPG Core page 549 says that if the item requires a specific alignment, reduce the price 30%. Well, the user doesn't have to be a specific alignment in your case, but still... the target does, which is close enough. Let's hack a third of the price off. From 1,200gp we drop down to 800gp.
End result, your
+1 dagger of demon-slaying and
+1 dagger of angel-slaying should cost 3,102gp each.
Not a big deal, numerically.
Mind you, a 3rd-level character should have 3,000gp to their name, and typically with character-building no more than 1/2 of a character's wealth should be invested in one object. I wouldn't sweat it
too much since I don't know how far through 3rd they are yet. At 4th, you should have 6,000gp, which would put them in a nice place balance-wise.
So, if these guys are at the bottom portion of 3rd level, they may be lightly over-equipped (or worse, depending on what other treasure they've got). If they're on their way to 4th, they're possibly right around where they should be.
A.K.A. take it easy on other treasure for a while and you'll be on track soon.
Incidentally, you should probably let your players know exactly what the nature of the enhancement is. As in, that it's not actually a +1 equivalent, but some additional ability worth 800gp that's tacked on. That way, if they elect to sell the weapons, or if they elect to further enchant the weapons, the math is clear. They sell for 1/2 the 3,102gp, or they add an additional +1 at 6,000gp cost as usual. The weapons will forever be 800gp more valuable than their pluses would suggest.