The difference is what you buy, not how you pay for it.
With custom development you pay for the whole site upfront and it's yours from day one. With the Starter Plan you pay €199 setup and €99 a month, plus VAT, and the site is built on a proven structure instead of designed from scratch. It ships sooner and costs less to start; in exchange, customisation has a ceiling.
If you need bookings, a catalogue, a private area or a connection to the software you already use, custom development is the better fit for you.