The worrying thing is that media buys aren’t the only thing a candidate needs cash for. There’s the ground game (get out the vote), legal fees for ballot challenges, etc.
Also, at some point, all the “free Trump publicity” is going to turn into “the mainstream media relentlessly attacking Trump, not giving him a chance to respond and rolling out the red carpet for the Hildebeast.”
At which point he may be wishing he had more cash in his campaign fund.
Don't be deceived by what Trump has in his campaign fund.
If he is self-funding, why move large amounts of cash into a campaign fund? He probably has a "just in time" method of replenishing the coffers. In the meantime, why not keep it in his personal accounts until he actually needs it?