I wouldn't say that the prediction is premature as the Heathrow website linked to in post #2 states the following:
There will be clear signage on roads in and around the airport to ensure drivers are fully aware of chargeable areas. Automatic Number Plate Recognition (ANPR) cameras will be used to enforce the charge and Parking Charge Notices (PCNs) will be issued to vehicle owners if payment is not received. Payment information will be shared ahead of the scheme launch.
As to the amount of the penalty charge.
The drop off fee at Gatwick is also £5 and the charge for non payment is £100 (reduced to £60 if paid within 14 days of the PCN being issued) so I wouldn't be surprised if the penalty at Heathrow is very similar to this.