I would think making sure outside payment links aren’t scams will be more expensive than that because checking that once isn’t sufficient. Scammers will update the target of such links, so you can’t just check this at app submission time. You also will have to check from around the world, from different IP address ranges, outside California business hours, etc, because scammer are smart enough to use such info to decide whether to show their scammy page.
Also, even if it becomes ‘only’ hundreds of dollars, I guess only large companies will be able to afford providing an option for outside payments.