I'm almost certain Pathfinder doesn't address this. Like nearly everything about Pathfinder mounted combat, you've probably come to expect that. "Mounted combat is underdetailed in Pathfinder," says creative director James Jacobs here. "So, the more you get into it, the more you're going to have to house rule." However, if you're willing to back up to Pathfinder's antecedent Dungeons & Dragons 3.5, that earlier game—if you turn your head and squint—can satisfy your desire for this level of detail.
Races of Stone (Aug. 2004) addresses the inequity of saddle prices and weights, but only in a footnote then only from extrapolation: "Weight given is for a saddle meant for a Large creature. Saddles made for Medium mounts weigh half this amount, and saddles made for Huge creatures weigh twice as much" (159), but nothing else is said anywhere. To be clear, this method of adjusting weight and not price conforms neither to the chart Armor for Unusual Creatures (Player's Handbook 123 and here and for Pathfinder here) nor to the explanation given as to how to adjust a weapon's price and weight for a creature's size (Rules Compendium 152 and Pathfinder saying only—so far as I'm aware—that a weapon's "price is the same for a Small or Medium version of the weapon[, but a] Large version costs twice the listed price" on this page).
I have to assume that Races of Stone's heart is in the right place: it's words are inaccurate but truthful, so saddle prices and weights in my campaigns receive the equivalent adjustments as per the column Humanoid of the chart Armor for Unusual Creatures (yet with the Medium row becoming the Large row and so on), those being what I think are the footnote's closest equivalent. Below are those extrapolated numbers that I use in my campaigns:
\$\begin{array}{|l|l|l|}
\hline
\textbf{Mount Size} & \textbf{Saddle Price} & \textbf{Saddle Weight} \\
\hline
\text{Small or littler} & ×0.5 & ×0.1 \\
\text{Medium} & ×1 & ×0.5 \\
\text{Large} & ×1 & ×1 \\
\text{Huge} & ×2 & ×2 \\
\text{Gargantuan} & ×4 & ×5 \\
\text{Colossal} & ×8 & ×8 \\
\hline
\end{array}
\$
An individual GM, of course, may rule otherwise, and, for example, forevermore force noble grig knights that are aboard their stately eagle steeds to ride those eagles bareback because a riding saddle always weighs 25 lbs., no matter the mount or rider. This player would think that's dumb, but he wouldn't fight the GM over it.