As for the costs, I couldn't tell you for sure but If you want to run a full node and connect it with your BTCPayServer, then you should see how much a full node would consume (blockchain size, bandwidth etc.) and buy hosting based on that. Or, you could simply deploy it to Azure using the button in the GitHub repository, Azure will give you 30 days trial with free credit, and monitor how much you're consuming and see.
Alternatively, you could use third-party hosting services but clearly, being your own payment processor and running everything yourself is much better, how profitable is it will depend on the amounts you're expecting to receive.
omg, thank you for your suggestion.
I have a success in setting up basic function like generating address.
Gona hack around to create instant forwarding address.
Need to create hot wallet and cold wallet instant separation by percentage.
I feel that I can squeeze the world in my hand right now
Thank you so much.