Afaik sweat price has been in steady decline. When I started it was 1.5, shortly before that it was around 2.0 and before that even up to 4.0 or 5.0 (i've heard).
The major changes in the price were due to
MA changes on the availability of sweat, e.g. increased "drop rate", removal of sweating cap etc. There was also a short-lived rise in the price once when
MA "fixed" the possibility of trapping mobs and sweating them.
I think, as you suggested, the price of nexus does influence the price of sweat (nexus price goes up, sweat price goes down and vice versa), but only to a small extent. I remember times when both of them went up at the same time and down at the same time.
The steady decline that has been going on even when there has been no changes to sweat/nexus availability can probably be attributed partly to the fact that MF skillers have slowly realized how uneconomical MF is even with ME at ~200% and they have realized that Hades(heim) will freeze over before
MA introduces some useful economical MF chips.
With sweat at ~0.5 I think it's getting close to the bottom, providing there are no changes to sweat availability - simply because people will stop sweating if it gets much lower...but that is just a hunch.
edited: was being a bit slow to write this post, so didn't see thoreau's, lol