Hi there! According to their website (https://www.tenable.com/products/nessus), it looks like Nessus offers the following pricing on a Professional license:
1 Year – $2,390.00
2 Years – $4,660.00 (Save $120.00)
3 Years – $6,811.50 (Save $358.50)
However, it appears (from that same page) that you can also do a free 7-day trial of Nessus Professional which allows you to scan up to 32 IPs.