Yes, it depends on the level, but they're not bad. For me, the Endless Scrolls first link doubles the initial production. For the Gum Tree, the original production is 116 and the first link is +87.
I should note that as a somewhat odd kind of UI design choice (in the browser): the production above the bar will show the links as well, with the value displayed in green, if you have any links that produce the same thing as the original production. But you can point the mouse cursor at the green number and see a break-down of the original production plus links. Yet if the links produce something different from the original production, they're not shown above the progress bar, only beneath it. So I think this could make it kind of confusing as to what you will actually get when you collect...but that's neither here nor there.
(As an example, the first link of Endless Scrolls produces Scrolls, so the two values get combined in the green number, but the second and third links produce Spell Fragments, and those aren't listed above the progress bar at all.)