You are using "Convert.ToInt32()" inside sql query, but that is not a sql inbuilt function, it is in c#. So, you can do like...
int pac_id = Convert.ToInt32(e.CommandArgument);
string sqlQuery = "update tbl_bid
set available_bid = available_bid + (select Package_Price from package where pac_id = '" + pac_id + "')
where lid = 1";
SqlCommand cmd = new SqlCommand(sqlQuery, con);
Make sure all your column names are correct and "available_bid" and "Package_Price" should have same data type.
I have created tables and columns as you posted when you first asked the question like ...
package(pac_id,package_bid,package_price) and tbl_bid(bid_id,lid,available_bid)
And the query worked in my case..