Year 1 of Full Serve is included with every perpetual license purchase. Renewal is required to keep receiving software updates and support; without renewal, the seat still works but stays on the version it was on when Full Serve lapsed.
Annual rental (subscription) products include Full Serve in the rental price — you don't pay for it separately.
Industry-standard maintenance fees run 10-20% of the perpetual license cost per year. Vertigraph's $199/yr Full Serve on a $799 BidScreen XL perpetual works out to about 25% of the license cost — a bit above the typical range at the BidScreen XL price point, but only about 7% on the SiteWorx/OS perpetual ($2,999).