When you arrive at Hummingberg for the first time, you might find that the prices are a bit higher than you expected. That's normal! Gwen should come over to help you out with this as part of her story.
If Gwen is not speaking with Theodore about his prices, you might have skipped a step in her questline that she's waiting for you to finish before she can start her smackdown.
Please return to your ship and check if Gwen has anything to say. If not, please go back to Barkensheim Creek (the island where Gwen taught you about chopping trees), return to the island, and check in with her again.
Once you find what she wants to say to you, she should go forward normally with dealing with Theodore.