Edit Unit Price line item After Adding product to the cart
this rule update unit price Unit Price of all Commerce Line Items to $200 when "Add a Product to Cart" Event is fired.

{ "rules_set_unit_price" : {
"LABEL" : "Set Unit Price",
"PLUGIN" : "reaction rule",
"OWNER" : "rules",
"REQUIRES" : [ "rules", "commerce_cart" ],
"ON" : { "commerce_cart_product_add" : [] },
"DO" : [
  { "data_set" : {
      "data" : [ "commerce-line-item:commerce-unit-price" ],
      "value" : { "value" : { "amount" : 20000, "currency_code" : "USD" } }
    }
   }
  ]
 }
}