It depends on why they don't have license plates on the car. If they were stolen, then it's about $10 for the fix-it ticket and $25 for the replacment plates. If they've never licensed the vehicle at all, then they'll get hit with a ton of penalties for the overdue registration, PLUS either the fix-it ticket or a ticket for non-compliance.
The penalty schedule is:
If payment is late: The penalty will be the sum of:
1-10 days* 10% of the Vehicle License Fee due for that year/10% of the Weight Fee due for that year, if any
11-30 days* 20% of the Vehicle License Fee due for that year/20% of the Weight Fee due for that year, if any
31 days-1 year* 60% of the Vehicle License Fee due for that year/60% of the Weight Fee due for that year, if any
More than 1 year-2 years 80% of the Vehicle License Fee due for that year/80% of the Weight Fee due for that year, if any
More than 2 years 160% of the Vehicle License Fee due for that year/160% of the Weight Fee due for that year, if any.
If it's a POS car with inexpensive registration, then the fees aren't bad, but if it's a relatively new or expensive car, then the added penalties really make a difference. (ex. $300 registration + 160% = $780)
If they're actually running without any kind of plates/registration, turn them in...it's the neighborly thing to do...