Creating products with Rules and calculating price in Drupal Commerce
Well It looks like I found a way. I needed to save an entity before adding to cart.

If someone will found himself struggling in the same spot, I will leave an exported rule here:

{ "rules_grafeon_doplnkov_slu_by_pri_pridan_do_kosika" : {
    "LABEL" : "Grafeon - Doplnkov\u00e9 slu\u017eby pri pridan\u00ed do ko\u0161\u00edka",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules", "commerce_discount_extra", "commerce_cart" ],
    "ON" : { "commerce_cart_product_add" : [] },
    "IF" : [
      { "entity_has_field" : {
          "entity" : [ "commerce-line-item" ],
          "field" : "field_doplnkove_sluzby_item"
        }
      },
      { "data_is" : {
          "data" : [ "commerce-line-item:field-doplnkove-sluzby-item:0" ],
          "value" : "Service SLA"
        }
      },
      { "commerce_discount_extra_line_item_has_product_type" : {
          "commerce_line_item" : [ "commerce_line_item" ],
          "product_types" : { "value" : { "product" : "product" } }
        }
      }
    ],
    "DO" : [
      { "variable_add" : {
          "USING" : { "type" : "date", "value" : "now" },
          "PROVIDE" : { "variable_added" : { "variable_added" : "Added variable" } }
        }
      },
      { "data_convert" : {
          "USING" : { "type" : "text", "value" : [ "variable-added" ] },
          "PROVIDE" : { "conversion_result" : { "conversion_result" : "Conversion result" } }
        }
      },
      { "entity_query" : {
          "USING" : {
            "type" : "commerce_order",
            "property" : "order_id",
            "value" : [ "commerce-line-item:order-id" ]
          },
          "PROVIDE" : { "entity_fetched" : { "entity_fetched" : "Fetched entity" } }
        }
      },
      { "entity_create" : {
          "USING" : {
            "type" : "commerce_product",
            "param_sku" : "DS-[commerce-line-item:order-id]-[conversion-result:value]",
            "param_type" : "doplnkov_slu_by",
            "param_title" : "Service SLA",
            "param_creator" : [ "site:current-user" ],
            "param_commerce_price" : { "value" : { "amount" : 100, "currency_code" : "EUR" } },
            "param_commerce_stock" : "1000"
          },
          "PROVIDE" : { "entity_created" : { "entity_created" : "Created entity" } }
        }
      },
      { "data_calc" : {
          "USING" : {
            "input_1" : [ "commerce-line-item:commerce-unit-price:amount" ],
            "op" : "*",
            "input_2" : "0.15"
          },
          "PROVIDE" : { "result" : { "result" : "Calculation result" } }
        }
      },
      { "data_set" : {
          "data" : [ "entity-created:commerce-price:amount" ],
          "value" : [ "result" ]
        }
      },
      { "entity_save" : { "data" : [ "entity-created" ], "immediate" : "1" } },
      { "commerce_cart_product_add_by_sku" : {
          "USING" : {
            "user" : [ "site:current-user" ],
            "sku" : "DS-[commerce-line-item:order-id]-[conversion-result:value]",
            "quantity" : "1",
            "combine" : "0"
          },
          "PROVIDE" : { "product_add_line_item" : { "product_add_line_item" : "Added product line item" } }
        }
      }
    ]
  }
}