How to increase the cost of an item using triggers

[Avengerbird123]

i was wondering how can i increase the cost of an item periodically. such as increasing the price of an item by X% or decrease by X%. I need this because i'm making a trigger that increases the price of an item with the increase of wealth of another player. thank you. sorry for my bad English. its about 2 am here.

Thank you for reading.