I have found large tubes of W&N Winton titanium white cheap on Amazon a couple of times. I've got three 200mL tubes at the moment just because I buy one each time I see it go on sale. If I were you, I'd sign myself up to the email lists of a couple of different online art supply stores. That way you'll know who has what on sale and when.
As I understand it, different colours can be different prices because some pigments are more expensive than others. Some brands will separate the colours by their price-points and label them "Series 1", "Series 2", etc.