I don't think it is wise to sell the plane with cheap servos in it, unless that is an understood part of the deal. Sure, if you just want to prove that the plane just flies, put Futaba 3003's or Hitec HS-311's in it. They are both around ten bucks. These servos are not strong enough for aerobatics in a plane that size, however.

Edit: I don't understand...you just want to prove the plane flies. But why then take the Hitecs out? Why replace them with cheap servos just "to show the plane fly with?"