Если не завязываться на количестве пробелов:
[{"key":"text","prices":["16.11","19.22","21.33"]},{"key":"text","prices":["17.11","23.22","25.33","27.55"]},{"key":"text","prices":["18.11","26.22"]}]
Если количество/вид пробелов жёстко задано:
[{"key":"text","prices":["16.11","19.22","21.33"]},{"key":"text","prices":["17.11","23.22","25.33","27.55"]},{"key":"text","prices":["18.11","26.22"]}]