Is the price listed for a specific magic arrow, such as a Sizzling Arrow (which costs 1,516G) the price for one such arrow, or is that the price for a bundle of 50, like the generic magic weapon rules states?
For one arrow. Fifty +1 arrows cost 2000 gp, so there's no way that fifty sizzling arrows would cost less.
Thats for one. The price supports it, as does the wording used in the weapon description.