Referrals save money in a few ways. External recruiting agencies can cost somewhere in the range of 15-25% of the candidate's first year's salary. You can see how referrals would make a lot of sense from just a financial standpoint. There are other benefits though - for one, referrals have a higher likelihood of responding to outreach, making it through the interview process, fitting in, and sticking around. I'm sure the referrer doesn't mind the extra dough either :)
In terms of referral bonus amounts, my company (PagerDuty, YC S10) offers between $5K and $20K depending on the role and priority. This includes engineering roles as well. The cool thing is we pay out for external referrals. Anyone can send a referral our way and if a hire is made, you'll get paid the full amount. Not too many companies do that :)