You're just starting out, your costs can't possibly be that high.
Maybe in two years you could charge $7 a month, but you're going to lose business doing it now. You also have no levers to turn to increase the price really, as anything close to $10 is nuts.
Obsidian is free and you can sync using whatever method you'd like. The $10 is if you want them to sync your vaults for you, all while being end-to-end encrypted. I've paid for sync since beta (I'm locked in at a lower price) and I partly do it just to support the devs making such a good product.
