The short answer
A single-purpose landing page — one page, one job, built to convert — typically costs $1,000–$3,000 as a one-time build, plus a monthly care plan for hosting, edits, and upkeep. Prodigy's public number is $2,430 to build and $585/mo for care.
If a quote comes in dramatically lower, ask what's missing — usually hosting, mobile testing, or analytics setup. If it comes in dramatically higher, ask what you're actually getting for it — most of the time it's agency overhead, not more page.