Spoke to Helen at Landcruiser park about Tough Tracks and their new pricing policy which is now the same across the board for all competitions held there.

Officials still get in for free
Navigators still pay full price
Drivers will now pay for ONE nights camping only

We had heard you had to pay for tow cars as well, is this true?

you would have to check with Helen but ...

if the comp driver towed in the car then no
if it was a truck then no
if the tow car is likely to be used to drive around in [say by family that came in the tow car with the driver] then yes

you really need to talk to Helen as each case is different

so, if reading between the lines, if the driver puts his family in the navi's car when they come in then it will be cheaper that way.