Is there an agency website platform with a one time cost?

Partly. Statamic and Craft CMS sell a licence for each site you build, paid once, and the site keeps running if you never pay again. Updates after the first year are $99 a year on either.

That is still a cost for every site, so the total grows with your client list. Our own system is not a one time cost either. It is a setup fee plus a monthly fee that is not charged per client site. Both prices are in the platform directory, with the date each was read.