What I usually allow is for the PC to choose arrows of the same Bonus of different types for their 50. So they may take 10 Flame arrows, 10 Evil outsider Bane arrows, 10 Shock arrows, 10 Cold arrows, and 10 contrust bane arrows. That way they have a variety of types of ammo to use. That's still 8000gp but if they combined them with a special weapon bow (+1 acid for example) then they are adding bonus of energy damage that make them quite powerful. It's an investment.
At higher levels, if you allow it, in Complete Psionic is the Quiver of Energy that generates energy damage for unmagical projectiles. It cost around 15750gp and holds 20 arrows that must be left in the quiver for at least 1 round.