Hence, my simplified rule of thumb for legacy items without penalties is that they should cost about one-third of the standard wealth that a PC should have at each level. In order to unlock the next level's ability, the PC should pay the difference in costs (ignore the cost of the mundane item).
Example: At 4th level, a PC's legacy item is simply a +1 longsword (cost: 2,000 gp, ignoring the base cost of 315 gp for a masterwork longsword). The standard wealth for a 5th-level PC is 9,000 gp, and one third of this is 3,000 gp. In order to unlock his legacy item's 5th-level ability, the PC has to pay 1,000 gp after he reaches 5th level. The standard wealth for a 6th-level PC is 13,000 gp, and one third of this is about 4,300 gp. In order to unlock his legacy item's 6th-level ability, the PC has to pay 1,300 gp after he reaches 6th level, and so on.