銀色の鉛筆は、高そうに見える。

[healer]

The translation is "The silver colored pen looks expensive."

Why would one need to say 高そうに見える instead of 高そうだ to mean looking expensive?

What the difference in nuance if there is?

By the way, can one simply say 高そう for the plain casual form?

 

[bentenmusume]

高そうに見える specifically emphasizes the "appearance" aspect of it.
I believe either Toritoribeさん or myself mentioned before that the -そう construction meaning "seems" does not necessarily refer to physical appearance.
If someone tells me they're majoring in quantum physics, or describes to me their recipe for apple pie, I could might say 難しそうですね or 美味しそうですね, respectively.

(If -そうに見える seems redundant to you, it's only because you're thinking in English and mentally glossing -そう itself as "looks" or "appears.")

And yes, the plain form is 高そう. You'll hear forms like this （美味しそう！美味しそうだね！） all the time.