They may have not paid for it
Prosthetics may be considered to be reimbursable by your healthcare provider.
If there was a big war (that could leave a lot of landmines), or some natural disaster that would cripple a lot of people ("thanks to our new safety systems, nobody died, they're just broken a little"), or just enough lobbying from the robot industry (which is actually very likely if it starts being cheap).
"Everybody has a smartphone"
Everybody, everywhere (actually not, but smartphone ownership spans over most social stratifications). Whether it's a low-grade one, a stolen one or a "I've-ruined-myself-but-it's-totally-worth-it" one.
If your social status depends on it and there is a way to get one, getting one is a chance to get out of poverty or at least forget it for a while. I don't think any poor person wants to be seen as such.
"Special" payment plans
A very honest prosthetic seller said this to me one day:
Here, I'm giving it to you for 10 $. Just sign this contract, oh,
don't read those tiny lines about you being indebted for life, they're
here for technical reasons.
I was not poor at the time. Now I am. I'm not allowed to sell my prosthetic back. I'm actually legally bound to publicly say that it is great and I could not live without it.