Google Custom Search has slightly expanded the range of services.
The Standard and Non-profit CSE services are unchanged.
They also offer an CSE via a JSON API: there’s no Google branding on that, but you pay $5 per thousand queries, and are limited to 10,000 search queries per day.
The new and fourth offering is a “Site Restricted JSON API”: it also requires the same “$5 per thousand search queries” payment. But if you search across no more than 10 URLs, then there’s no daily traffic limit.
I guess a use-case for this would be a huge and very heavily-used corporation like Boeing, where you want to offer your clients the quickest and most accurate way to search across all your technical reports, papers and manuals — which are spread across 10 different URLs? That use-case would likely need some guarantees from Google, though, on the spread and depth of the indexing.