Titanium dioxide is used in virtuallyeverything that has white pigment.  Its value proposition is mainly thatit is nontoxic and highly effective at producing opaque white color, and it canbe found in food, coatings, and consumer products.  It is also highlyreflective.
      If your question focuses on pricing, I have very little to offer; the price ofa ton of titanium dioxide has been especially volatile in recent years becauseof changes in demand and the price of source minerals such as rutile orilmenite.  There has also been a lot of capacity expansion over the pastten years, particularly in China.
There is also a dichotomy in titanium dioxide pricing based on productionprocess.  Sulfate process titanium dioxide is generally cheaper thantitanium dioxide produced by the chloride process in part because it requireslower-quality and thus cheaper feedstock* (ilmenite rather than rutile oranatase) and in part because chloride process titanium dioxide is preferred forhigher-end applications.  Until recently, the sulfate process wasconsidered to be slowly dying as old sulfate plants in the US and Europe shutdown due to uncompetitiveness, but that story has reversed itself due theChinese building a lot more capacity using the sulfate process over the past 10to 15 years.
      The main substitute for titanium dioxide is zinc white.  Titanium dioxideis much more valuable because it produces a brighter white color but zinc whiteis sometimes preferred for price reasons.  Another substitute islithopone, a mixture of zinc and barium salts.  Bleaching could also beconsidered to be a "substitute" in certain applications, such aspaper.  The application area for titanium dioxide is very diverse so it isdifficult to name specific substitutes without specifying the application.
      Below is a graph showing the different refractive indices of titanium dioxideand substitutes.  Rutile titanium dioxide is generally produced by the chlorideprocess, while anatase is a lower-quality titaniferous mineral that is usuallyprocessed by the sulfate process.

At least, for traditional chlorideprocess.  DuPont's titanium dioxide division has a chloride process thatcan use the lower quality feedstocks once restricted to sulfate processproducers.