The meaning of "がい" in 落としがい

I think a little more context for that sentence would help, but basically this is the noun 落とし followed by a form of the phrase ～甲斐{かい}がある, which means 'be worthwhile'. It's an N2 grammar point (see here). The かい part often becomes がい due to rendaku. That is why it is がい in おとしがい.

As for 落とし, one meaning is 'getting someone to like you romantically'. I'm assuming this sentence is something to do with a relationship. If so, 落としがいもありそうだ would mean something like "It seems like it would be worthwhile if that person fell in love with me". Again, this interpretation might be different if the context is different. But the point remains that the phrase is a combination of the noun 落とし with a form of the phrase 甲斐がる (with rendaku).

The phrase is mentioned in this article about things about girls which make guys think it wouldn't be worthwhile to be in a relationship with that girl. The point of the article is 男が「落としがいがないな」と思う女の特徴4つ.