Don’t worry about not getting a lot of sales right now, you are not the first nor the last to be struggling with this scenario, 80% of the visitors don't place an order on their first visit.
Your store seems really good and professional and really like how you add the Christmas touch to the logo. Regarding your question, there’s no reasonable average cost per purchase.
Since Facebook ad is a bidding system, the cost fluctuates every second. The cost is higher during peak season. The cost is higher if your product price is higher. The cost is higher when you are targeting some countries such as the US, because everyone is trying to sell their products to the US!
Check out this guide for Facebook ads , hope it helps you out.