Each Collectible they accept has three ranges. There is a "low" range, that offers the least Scrip and exp (and Gil), there is a "medium" range, which offers more, and there is a "high" range, where they offer the most exp, Gil, and scrip.

There is, however, a minimum Collectability that they will accept. If your Collectible is under that minimum rating, they simply will not take it.