So when the Trading Post comes out soon, I think it would be neat if accounts with no game time could earn maybe 3 - 5 days of game time each month through the new system. This will give F2P players a small window each month to farm gold to get a WoW token so they can continue playing for another 30 days.
Game time earned from the Trading Post should only be earned once a month however. Perhaps the cost of 3 - 5 days of game time should be about 300 trading tendance as I imagine it would be quite difficult for a F2P player to earn 500, because there will likely be less objectives they will be able to do at level 20.
Also F2P players will not be able to get their 500 free trading tendance every month as you need to have game time to collect the 500 you get just by logging in in the first place.