Okay, simple fact is, you'd get a TON more people subbing to Nitro if there were a) cheaper options, and b) if the perks made any sort of sense.
Emojis, custom URLs, etc. - these are fluff. They are not practical for most people. There is no FUNCTIONAL use to them. The emojis, I can almost guarantee that granting a server 50 more emojis does not take up a significant amount of bandwidth, nor does it require additional coding. It's already a feature you get for free, just *more*.
However, better audio quality? Better video quality? Yeah, those cost bandwidth and/or server space. The ability to customize notification sounds or badges for your server, so that, say, a message on your server notifies the user with a specialty sound or a blue badge, not a red one? The sounds would take up additional server space, and the customizable badges would take a little additional work.
I'm not going to try to rehash all of the great suggestions people have. Suffice it to say there are TONS of solid suggestions out there. You still don't know what people want? Curate your suggestions box, and ask users to take a poll. You'll get your feedback.
Regardless. $10/mo MINIMUM is ridiculous. I'd be happy to pay $2/month and get NO additional benefits, just to support a really solid program that I LIVE on. $5/mo for some USEFUL features would be great. But if I want to get a server I'm hosting or participating on to a point at which our shared audio quality is great? I can't do it alone. And I can't do it for cheap. And I can't pay a one-time fee to permanently add one or two necessary features to a server.
Yes, fine, the multiple boosting system makes SOME sense on the huge servers with hundreds or thousands of participants. But I spend the majority of my time on a server that has never had more than 15 people. We game together (remember, this is a gamer thing, right?), but we have people from all over. The audio quality sucks, when you compare it to what was available with Mumble or Vent or TS. There are constant outages or glitchy messes, and the sound is rarely balanced between sessions.
That's just my example - everyone's different. Point is:
TL;DR - Provide affordable, useful benefits, and more people will gladly shell out the money.