Relatively, they're correct. Compared to other app stores like Apple where the fee is $100/year, which personally I'd consider huge. To get on Steam, the fee is a one-time $100 with the ability to get it back after $1000 in sales. It's even low compared to Google's other marketplace, the Play Store, which has a $25 one-time fee.
While $5 can be a lot for certain people, relative to the market it doesn't deserve the title of being "huge".
Once you paid the fee, you can publish as many extensions as you want.
https://developer.chrome.com/docs/webstore/register/