As far as I'm aware there's no such rule, its just a popular houserule.
However, the DMG (page 199) offers a few suggestions for starting items for characters higher than 1st level.. one of these is to limit the player to 1/4 of his total on any one item, so that would be 50k in this case. Another suggestion is the item by item veto, or also you can limit a player to only having access to 'minor' magic items, etc.
It also has some decent guidelines on pcs starting off with charged items and magic items they created themselves.