I priced it at $3 in comparison to Lookout. Lookout can let you choose a card to trash, and also lets you cycle and filter; this doesn't give you a choice of card, but upgrades as it goes; they both can be unlucky late-game and force you to trash a good card. You think as written it's too strong for 3? Hm, maybe. Since costs in the 3–4 range for trashers are mainly based on early-game utility, let's look at effects on the first shuffle:
If you open Lookout/4, to a first approximation, on the first shuffle you get:
c. 34%: trash Copper, discard Copper
b. 51%: trash Estate, discard Copper
a. 15%: trash Estate, discard Estate
If you open Augment/4, you get:
d. 64%: trash Copper
e. 27%: trash Estate, gain Silver in hand
f. 9%: no thinning, but retroactively convert your 3/4 open into a 3/5 open
(e) and (f) are both potentially pretty good, but are they good enough to compensate for the fact that 64% of the time the first-shuffle effect is strictly worse than any first-shuffle effect of Lookout?
Here's Upgrade for comparison:
g. 12%: trash Copper
h. 88%: trash Estate, gain Silver
I guess I don't know how to compare the fact that Augment has a better best-case scenario with the fact that its best-case is so much less likely than Upgrade's. (Well, without playtesting, anyway.) I guess what that does is it just makes Augment really swingy, which might not be a lot of fun to play.