There is not a price for away fans.
Basically if it's on the website and you can book it then you can have it.
For the Hull game the East terrace is already on sale. But you have to be aware, if you are wanting to go as a group of away fans booking early may mean you are not with the rest of your supporters.
Basically it will depend on how many tickets are booked pre the away club (in this case Hull asking for an allocation).
As far as I am aware most away clubs get their allocation about a week before. For example on the Saints website you can get home match tickets up till the hull game on the 9th March. But the Giants game tickets are not up yet on the 4th March.
So the section allocated to away fans will be dependant upon what amount the away club want and where the remaining space can be found to allocate that amount.
ie If Hull want 1000 tickets and the East stand has 1000+ tickets left then the allocation sold at the KR will be for the East stand. But if tickets sold for the East stand only leave say 800, then someone will have to decided do they offer the 800 for the east stand and 200 elsewhere or do they just do 1000 elsewhere.
This is the point I was trying to make about Wigan and Warrington getting less than usual, it's not to put anyone nose out , but sales will be first come first served.