How honest to be with your future employer

I've got a job (dead end job) the pay is awful. Got an offer for another company turned them down now we are negotiating ( a little better pay still bad) The manager and corporate office are worried that if they give me more money than they usually do for this position ill be gone in the year for a better job ( which is the plan). Do i tell them what they want to hear or the truth
They have expressed that Iam the top candidate and they really want me if I seriously want to stay a while