First, I would not worry about whether or not the local transport agency was getting short changed- it's supposed to be about what's in the patient's best interests. Issues like that are for policy makers to hash out.
I would be very careful about your policy. First, how do your approach these nontransport calls? What do you tell the person? How do you evaluate them- V/S, lung sounds, EKG, pulse ox, glucose, etc? The patient will always ask something along the lines of "Will I be OK to go by car, and not an ambulance?" That's a loaded- and very dangerous question, which requires an honest answer.
Obviously the situation depends on their chief complaint, but often times their stated problem is only the tip of the iceberg, and they have underlying problems that DO require immediate evaluation. We simply do not have the diagnostic tools or training to rule out many problems. Blood work, Xrays, CT's, ultrasound, etc. You may be putting your license-and your employer- on shaky legal ground by encouraging or implying a person does not need an ambulance.
Obviously someone with a hand laceration where bleeding is easily controlled does not need an ambulance. They may need sutures, a tetanus shot, or even an XRAY, but someone with a vague complaint like abdominal pain can be anything from gas to a ruptured appendix.
I do understand your concerns- especially in this economy. Ambulance service is EXPENSIVE, not everyone's insurance will cover it- especially if it is deemed not an emergency, but that is NOT our call. If you are willing to wait with a family for them to arrange a car transport, then clearly this is not an issue of getting back into service quicker and not wait on an ambulance. Anyone who refuses transport needs to be advised fully of potential risks(however remote), as well as the costs. It HAS to be the decision of the patient and/or their family. As long as they are fully informed- and not coerced into making a decision- then you should be OK as long as you accurately document and converse with medical control.
You will never get in trouble for encouraging transport, but you CERTAINLY can if you do not transport someone who turns out to be very sick.
Example: Years ago, I had a patient(round 30 y/o) who's family called because she was complaining of a head ache all day, and was just laying on the couch. The patient had no idea the family called us, wanted nothing to do with us, and was angry at her family for calling us. Her vitals were normal, although she was a bit photophobic, so I said she could have anything from a simple head ache, to a migraine to a CVA. I told her we would take her to the ER- it was literally about 6 blocks away. She refused, said she would call back if needed or have her family take her to the ER. Documented the run, called it in, and never gave it a second thought. A few hours later, we received a call from another crew who was at the local ER. They heard this patient's family in the ER, extremely upset- apparently her head aches got worse, they brought her in via private car, and she promptly had a seizure at the ER and went into a coma. Turns out she had a subarachnoid bleed and died later that night. We covered our arses, but of course the ER's first question was why we did not transport earlier. We arrived later at the hospital with another patient, I explained to the staff what happened, and they understood. Would the patient's outcome have changed if we transported earlier? The doc at the ER said probably not- it may have simply delayed the inevitable.
Point is, sometimes the simplest things can turn BAD, which means encouraging people NOT to use an ambulance can have very serious- and unexpected- consequences.