It really depends on how the 75 models are divided. If the models had a good selection of all sorts of trees (which I'm mostly interested in), and only some bushes, rocks, grass and flowers, I would pay around $10 for it.
Why not more? The problem with these kind of packs (even my own pack) is that I would use it now and then. Since FPSC isn't build for large outdoor level, but you can incorporate it if you build your level cleverly enough, I wouldn't create a game with lots of outdoors levels. With my current game only 2 of the levels are outdoors.
I'm not sure how much times goes into developing this and do understand that you don't give out your secrets, I don't either for my landscape pack and that's the only good working one around. But looking at this, I don't think it took ages to create these, so I'm saying around $10 would be fair.