The cost to start an online store splits into four buckets: platform and tools, product and inventory, getting found, and the costs nobody warns you about. The first bucket is small and predictable. The second is where almost all the money goes, and it varies...
The way to find a product to sell online is to run a filter, not to wait for an idea. You generate a wide list from places where real demand is already visible, cut it against fixed written criteria, then test the two or three survivors with a small amount of money...