The ‘wealth‘ referred to here is the company the daffodils give to the poet and the pleasure it brings to his mind when he is in vacant or in pensive mood. 
 
When Wordsworth saw the flowers in the valley, he hardly realised its worth. He then did not know that the sight of those daffodils would leave so much impression upon his mind.

I gazed and gazed but little thought
What wealth the show to me had brought.

But later, he has discovered that when he lies on his couch in lonely and thoughtful state, those flowers flash in his mind’s eye and fill his heart with joy. Thus the poet has come to know the worth of the flowers.