So, just in case someone finds it useful, here's what I ended up doing.
I take daily snapshots of each logstash index. Then, when I need more room, I create a tgz file of that day's snapshot, and archive it elsewhere. That lets me delete both the indices and the snapshot.
I save the metadata- and snapshot- files, then read which indices are in the snapshot, and find those directories in the repo/indices dir and add them to the tgz file.
If I need to access the data later, I can restore it into a temporary clusters repository and restore it from there into the temp cluster. Or just do the same in the prod cluster.
Of course, it will be interesting to see how well this works when the snapshot is from 1.x and ES is at 2.x...
And I haven't been doing this long enough to see if it really does what I want. We'll see.
Also, as @Aaron_Mefford said, it'd be better to adjust my requirements to fit my budget. Or the other way around. His other suggestion of removing specific kinds of data is also good. Though I'm going to try and avoid implementing it if possible, it seems complicated... 