In my experience, most halfway respectable companies already do?
500 bucks is IMO on the low side for an employee referral in the US if it's for software engineering... the amount of money you save the hiring team alone in recruiting efforts is often in the thousands of dollars, and they often stay longer and are often the best hires - not surprising if former colleagues they like are already there...
For roles I've worked, I've never seen less than $2k offered for a successfully recommended technical hire, both at startups and BigCos. Most places I've worked have offered around $5k.