Have you tried hearos? They're expensive but, well worth it. They're earplugs that retain sound quality a lot better than regular ones do.
To be honest; I tried expensive earplugs in the past and no matter what filter I put in, they all still sounded like earplugs. Muffled and weak sound with intransparent highs. Maybe it's my ears canals being too narrow (had a bad infection there in the past), but earplugs just don't cut it for me.
If he doesn't want to spring for plexiglass because its too expensive, he most certainly won't want iem's.
IEMs for the whole band are easily in the 900-1000€ territory. Unfortunately, this is not an option for us.
The problem with the Plexiglass walls is not that they are too expensive, it's that it would be a "drummer only" item, so I can't justify buying it (as we have a "everyone provides for their own equipment" rule).
We rented a plexi shield for a recent lower volume gig and it worked great for limiting the drum volumes into the room.
Did it sound muffled? I expect a severe change of room acoustics ... was it for the better? I'm a bit sceptical about drum shields because I read a lot of reviews where they just plain didn't work.
As much as I like the idea: E-Drums are not an option either. Our drummer already has an E-kit at home, but transporting it back and forth and setting it up every time would just waste too much precious time on rehearsals. ... and it's hard to justify a second E-drum kit for anyone.
Thanks for all the comments, everyone!
The JamHub thing came up multiple times... seems like headphones is the best budget solution so far. At least it's worth trying before we apply any other physical means (as it effectively costs me 0$ as our board has sufficient Aux Sends).
I'll report back in this thread after a headphone run this weekend.