Quick help with sentence structure.

How would I say

"I suppose that I should have expected it to result in sadness" ?

Je suppose que j'aurais dû m'attendre que ... dans la tristesse.

The expression for "to expect something to happen" is "s'attendre à ce que" and is followed by the subjunctive.

So you would get:

Je suppose que j'aurais dû m'attendre à ce que cela finisse dans la tristesse.

For "finish in sadness", it does depend on context, of course, but it would be more idiomatic to say something like: à ce que cela finisse mal / à ce que cela finisse en larmes.

It's a creative response to Sagan's Bonjour Tristesse, so I wanted a link back to the title.

In the end I went for:

Néanmoins, je suppose que j'aurais dû m'attendre à ce que le résultat ne serait rien d'autre que la tristesse.

Does this make sense?

It seems too wordy to me :/

No, that's very good - your serait should have been a soit, however, because s'attendre à ce que triggers the subjunctive. But even without it it's perfectly understandable.