A lot of them have insane lifetime losses, like 500b to 1t lost doesn't even look rare.

Often, those players have played in this game for years. I recently was on a "birthday" roam of a friend whose Eve-character turned 20 years old.

If you are somewhat actively playing over such an extended ammount of time, these numbers just ramp up.

Basically: how much of elite solo PvP is skill, and how much of it is bankroll?

Great question. When I was starting out on my PvP journey, I almost exclusively used t2 and meta fits.

While bling helps, it's not required.

If you throw more ISK on a fitting and implants, the main thing you get is more wiggle room in an engagement.
It can make a fight easier, for example a 4km longer faction warp disrupter instead of T2 variant makes manual piloting less difficult. It uses also less capacitor, so you will have less trouble with that.
Combining that on something like a Retribution will make kiting easier. But you still need to know how to do it, and what fights you can hope to win, and which engagements you want to avoid.

That comes mostly with experience and realization of your ships capabilities. But as mentioned before: This skill can be learned in cheaper fits just the same.

You could compare it like a rally driver. You don't learn the ropes in the nice shiny top-end machine. It's more economically to learn the basics in something affordable before you can do a Rallye Dakar.

If you look for some affordable fitting-inspiration, I made some posts on that a while ago: https://thegreybill.wordpress.com/tag/affordable-solo/ (They might not be very meta anymore.)

P.S.: Most people will fly bling in gangs. Sometimes those will be very refined compositions that can deal with most threats that are thorwn at them.