OPT One vs Sanity

OPT One

$129 one-time / user

No extra cost per site
$6/mo per extra collaborator
Hosting billed by your provider

Sanity

$0 free / $15 per seat/mo

3–5 seat team: $45–75/mo Sanity alone
Larger teams: $150–750+/mo
Frontend hosting: $0–50+/mo extra
Engineering to build Studio: often the largest cost

Sanity pricing can be fair for what it is: a content backend. Budget the frontend and integration work too.