A lot of people will say an expensive paddle makes a huge difference, and they advocate spending 300, 400, 500 or more to get a 'decent' paddle. I would argue there are 3 tiers of paddles and the middle tier is actually good enough for most people.
At the bottom are aluminum and wood paddles (not the Greenland ones, those are special). These are fine for fun at the lake or a rental for an hour or two. They're cheap and heavy. These are the ones I think you should avoid if you're buying a kayak.
In the middle are the basic fiberglass shaft / nylon blade paddles. I have one from Arts-Outdoors that cost about 80 and weighs a little over 2 pounds. The Aqua Bound fiberglass in the US is a little more expensive but the same idea - it's light enough that you can use it all day, and it's not too expensive. (If your arms get really tired using one of these, your technique might need work).
At the top are the fancy carbon or fiberglass options. I got one of these last year. It is really nice. It is about 20% lighter than my midlevel paddle. It's stiffer, and definitely more satisfying to use. I will keep it (and I'm taking it out later today). But it's not as much a gamechanger as everyone would have you believe.
If you are new to kayaking, I would suggest starting with the midrange paddle - something around 2 pounds/1kg. It will keep you happy for years.
What have the rest of you found? Are you enjoying your midrange paddles? What really makes a difference for you?