A safe formula to use for scale figures is:
- If it's a recent GSC figure, most likely it will go down in price or stay the same because GSC tends to overstock. But the best GSC figures of popular characters may go up despite that, i.e. the famous Madoka item #98665. Recently though, GSC has been doing quite a few re-releases, so if a new season/movie of an anime is coming out, hold out on buying the characters' figures in the aftermarket and watch out for re-release preorders!
- If it's Alter, most likely it will go up in price or stay the same post-release, because Alter figures are expensive to begin with and highly sought after. Alter figures of very popular characters, i.e. Love Live girls, are the ones in most danger. If you really like an Alter figure, just preorder to be safe.
- If it's Kotobukiya, it's rather random as Koto always has some sort of QC issue but likes to make less popular characters or unique poses. For example, Koto's Shiro figure item #218050 went up in price despite having some serious issues with her hair. Most likely it's because she's in a 'special' pose that other manufacturers don't usually go for and she's a beloved character with only 2 figures.
- If a figure only has a few or one manufacturer, it will mostly go up in price in the long term. This tends to happen with Megahouse' GEM figures even though their quality isn't the best. This is because Megahouse has the monopoly for a lot of Shounen anime. An example would be this Noragami Yato figure item #198458.
- If a figure is made by an 'unknown' or small company, it would very much depend on the distributed item. It's hard to predict until the item is released and people can comment on the figure since they don't always meet the expectation of their prototypes.
- If a figure is exclusive / limited, logically speaking there's a very high chance the price will rise. I'm guessing this would happen with TOM's Wedding Holo figure item #485674 because she's made to order and also TOM's first figure as a manufacturer. There also aren't much high quality Holo figures as of now, although that may change with the anniversary celebrations.
TL;DR when a figure is overstocked or overproduced, it will most likely go down in price. Hestia, for example, used to be the Waifu of the Year but almost all her figures binned because so many companies were making her. There's a high chance the same will happen for Rem figures coming out this year and next year. On the other hand, figures that are very limited (either because they're exclusive or because only a few companies made them) tend to go up in price. But it's always best to judge with your own eyes. An AMAZING figure will always have high demand and go up in price despite all this.