Kyrail said:
Doing it one arrow at a time breaks the whole reason to ever buy keen arrows.
Frankly, I think you'd be hard pressed to come up with any reason to buy
keen arrows.
They must be at least +1 arrows to begin with, and
keen adds another +1. So, we're talking 50 +2 equivalent arrows.
This results in a cost of (50 * 7gp) + 8,000gp = 8,350gp, or a cost of 167 gp per arrow. In other words, every time you shoot, you are throwing away a 2nd-level scroll. For a decent archer, this means you are throwing away 300-500 gold pieces *per round* (on 3 to 4 attacks).
Oil of Keenness, under the proposed house rule, has a cost of (5 x 3 x 50gp) = 750gp, or a cost of 15gp per arrow. Tack on another 1gp per arrow for an Oil of
Magic Weapon, and you've got the same effects as above.
Whether it lasts for one battle or for many, actually enhancing ammunition is a bad, bad idea (from a cost perspective).