Hi I don't know what the cheapest scanner is that has bi-directional functions, but I don't think that one with bi-directional is going to be cheap. I just bought the Autel MP 808 from Amazon and it has bi-directional . But the vehicle must support bi-directional too. A good way to go about it is to take your time and read all the info on each scanner to find one that is in your price range but the more features it has the more it is and there are many out there to choose from. I hope that helps. Frank