What's a good way to get into background acting?

[Cogito]

Extras agencies usually do charge a fee.
This is because their clients don't earn enough for them to take a percentage, as real acting agents do. Extras get paid minimal rates - often just enough to cover their transport costs to get to the location - so the agency has to charge people to join, or they wouldn't make any money at all!
Typically, an extra will only get called once or twice a year, just for a day or so.

I hope you realise that being an extra (or background actor) is NOT acting. They're just human props, and it won't help you get into real acting at all.