Travel from london (VIC) to kent canterbury (CBE) by train?

I would like to know if the tickets have a standard price or the price varies according to the time of purchase?? I would like someone who uses this line frequently to help me! Also if the saver day ticket has any time of travel restrictions in case i miss my journey??
Thanks

The prices may vary based on the time of travel, not the time of purchase.

Yes - "saver" means that its reduced price, but has restrictions.