Going off the main clause I think the general meaning here is that Saegusa (2nd person) is giving the speaker (narrator, 1st person) a surprised look (lit. "giving me a great suprise" - the speaker used 上品に to stress random things before so assume it's the same here).
(NARRATOR, 1st person): まあ、なんてこれまた上品に驚いてくれる三枝さん。
There's not much relevant context here other than the narrator just having surprised Saegusa by revealing a surprising fact (surprise!).
What I'm missing here is the role of なんてこれまた.
The grammar book I'm using as reference has two forms of なんて, the first one (a fixed phrase) used before a noun or a verb as a question, e.g.
Or as なんて＋Ａｄｊ｜Ｎ＋だろう as an exclamation, e.g.
The second form (a particle) is used after another word "to express a strong feeling", e.g.
As for これまた, I am at a loss. Does it carry any nuance? Google search didn't help much (or, to be more accurate, I don't really know where to look for).