I just bought a lifetime pass
The lifetime pass was $180 (I got a loyalty discount, woot!) and I figure I'll be playing Elite for the next year or two. Let's call it 30 months. So that comes to:
$6/month.
My previous estimate was that with the new expansion it'd bump up to $7/month so now I've basically locked in my monthly cost and if I hang around more than 30 months then my monthly rate begins to dip below $6/month.
Edit: Apparently the lifetime pass may also include Beta Access. Which makes it extra super tasty, because now I am one of the cool kids.
Of course I am gambling. I might get sick of the game (I'm vastly more likely to get sick of the forums!) and quit but I could also come back in 3 years or whatever and not pay. That means that if I'm curious what's up on ED land in 2020 I can drop by and check. Seems like a good deal and I'm willing to risk the money.
And here's another really great reason to do it (which is what clinched the deal for me!) in a year or so, when lots of people on the forums are screeching about the next expansion and what it costs, I'll be smugly inflation-shielded and prepaid. While my $6/month is fixed, now, theirs isn't. It might go up to a whopping $8/month and I'll get to sneer smugly as everyone pulls great handfuls of their hair out and rends their garments. It's worth it for that pleasure, alone!