Price is necessary because the rules for crafting items (both magical and non-magical) use the GP value of the item to determine xp cost and time to craft.
DMG gives price for a DC11 sleep arrow +1 as 132 gp: -7gp for the masterwork arrow leaves 125 gp magic cost: -40 for the +1 leaves 85 as the sleep cost: x50 gives 4250 gp as the cost to enchant a bundle of 50 arrows with sleep. (Magic ammunition are calculated in bundles of 50.) Where this number comes from is a good question. The RAW doesn't really handle adding "piggyback" spell effects onto magic weapons. Cost for a single-use, use-activated, slotless item of sleep ("dust of sleep" or the like) would be 100 gp. The sleep arrow as written actually has a weak Merciful quality as well (non-lethal damage, but without Merciful's +1d6 bonus), but that might be handwaved as "flavor" since presumably the whole point of such an arrow is to take an opponent out of the fight without killing them. The arrow effect is unlike the sleep spell in three ways: it's single-target instead of a 10' radius burst, it affects the target regardless of hit dice, and it lasts until they awaken normally, instead of 1 round/level. The longer duration and removal of the HD cap, IMO, more than make up for limiting it to one target, so this is probably equivalent to a spell higher than 1rst level. Deep Slumber raises the HD cap to 10, has a shorter range (!), and is 3rd level. The pixie effect is definitely better than Deep Slumber.
The amnesia arrows are quite powerful. The effect is permanent, can only be cured by Heal, Wish, Limited Wish, or Miracle, and causes the victim to forget everything, whereas Modify Memory only affects 5 minutes of memory and is a 4th level spell. A better prereq would have been Feeblemind (5th level).
DMG assumes all item saving throws are calculated off 10+spell level+minimum ability bonus, hence the DC 11 for the sleep arrow above.
spell level ... DC
1 ... 11
2 ... 13
3 ... 14
4 ... 16
5 ... 17
6 ... 19
7 ... 20
8 ... 22
9 ... 23
Now, the pixie arrows described in the MM have a DC of 15, which is impossible given the progression above. But it also describes the DC as "Charisma-based, with a racial +2 bonus". In other words, the arrow save DC is set by the pixie firing it, not by the arrow itself!
I see two ways to handle this. One would be to price the bundle of arrows as if making a staff (since staves are the only item to use the wielder's DC). The other would be to assume that the arrows are just normal +1 arrows and that pixies have a racial ability to "enhance" the arrows they fire, like an arcane archer, perhaps requiring a minimum Cha of 13 for the sleep effect or 15 for the amnesia effect. I strongly lean towards the latter solution, as it wipes away a lot of red tape - no need to calculate creation costs for the arrows, no "pixie supplies entire party with amnesia arrows" problems... it might up the LA by 1, but pixies have a high LA anyway (the "improved invisibility at will" power is worth a major LA boost all by itself) so anyone playing a pixie character has already shown a willingness to take a major LA hit. IMC, I would use the +6 LA and say that a pixie with a CHA of 13+ can fire one sleep arrow per round, 15+ can use Irresistible Dance 1/day, and 17+ can fire an amnesia arrow 1/day. I also would allow Greater Restoration to remove the amnesia effect, since it can cure Feeblemind, which is arguably equivalent if not worse.