Search conversion rate is the conversion rate of sessions containing at least one search. It is reliably higher than site-wide conversion because searching declares intent. The number that matters is not the absolute rate but the ratio between search and non-search sessions on your own site.
Absolute conversion rates vary enormously by category, price point and traffic source, so benchmarks are close to useless. The search-to-non-search ratio is internally comparable and moves for reasons you control.
Pair it with search usage rate. High conversion and low usage means the search bar works and nobody can find it — a visibility problem with a very cheap fix.