Direct answer
Every claim is sourced below
Lean Labs' article on the three stages of growth-driven design lists $30,000 to $70,000 and up for the first build, called a launchpad, and about $5,000 a month for the improvement work that follows. That is one arrangement: a one-time fee, then a monthly one. An agency can also bill the whole engagement as a monthly retainer, with no separate build fee. In either arrangement, the price follows how much the first build covers and how much work each month buys.