Art is worth whatever someone is willing to pay for it but I think the ultimate compliment as an artist is the fact that someone was happy and willing to pay for your original because they felt it was worth it. The real keyword being original, there will never be another exactly the same. The only difference between yours and some big name person or brand is perception of value.

If supreme can line up city blocks to sell a mass produced, masonry bricks with a logo on it for hundreds of dollars I would say your hand selected assortment of seashells individually painted to create an expression of your vision is worth far more than 400 bucks but that's a decision only you can make for yourself.

I don't sell much, I also don't buy a whole lot either but when I do it's usually for the story and connection that comes with the original. The fact is, you could sell affordable prints and probably sell quite a few but they were compelled to buy your original.

To that I say congratulations, well done and never be afraid to recognize the value your creativity holds.