That might be one logical approach, but it's not actually true. "Flight tested" rockets already offer a 10% discount to customers, and they say the plan is to get to 30% eventually.
In any case, considering the four-month refurbishment the reflown core may actually have ended up more expensive than a new one. That's okay for now. SpaceX offered a discount to get a customer to fly on it instead of having to pay the launch cost completely on their own. Currently I wouldn't say the discount is any indication about how much cheaper reflying a core currently is. But with the first stage being about ¾ of the launch cost there are of course great potential savings, which is where the 30 % come in.