Retainers exist because SEO is continuous: the site changes, competitors move, engines update. A typical retainer covers a fixed number of days or deliverables per month, a monthly report, and a point of contact. Prices range from a few hundred euros a month for a freelance consultant to several thousand for an agency team; the guide on costs on this site gives sourced figures.
What to check before signing: what is delivered each month in concrete terms, who does the work, how results are measured, and the notice period. On SeoGeoMap, an expert's listing shows the monthly retainer it declared, when it declared one; nothing is estimated.