The price input script is interesting, tho it would need to be modified to work with the default resolution. As for the image linked to in the instructions, there's no need for the $scene = Scene_PriceInput.new line, but there is a need for a wait event command between the Percentage variable condition and the PriceInput.Input(x,y) scriptcall. Otherwise said variable isn't changed when the conditional branch is read.
It should also be mentioned in the instructions that any items you designate by the scriptcall needs an icon image in the Graphics/Icons folder, with the same name as the item. As well as the image icon_back placed in the Graphics/System folder.
That's what I got when I tested that particular script on it's own anyways, makes me think of Recettear. Anyways the other resources are good too! Now to give Stella Sidus a proper test, first time I tried it I got some error when I talked to a sign, the 2nd time I tried it the game froze when I tried going from one map to another(think I re-entered the building you come out of and came back out?) 3rd time's the charm!