A company I know well cough is built an app and pays about $7/download via Facebook ads. This company has tried many other options and while it has gotten more conversions using alternatives, the users have tended to be poorer in quality (much lower DAUs/MAUs). I have heard the average cost/download via Facebook is lower than $7 (maybe $5) for an app like this one, but I think it has been worth the money spent, and I would/will do it again. And yeah this company has spent quite a bit more than $1k on these ads.

Downloading a free App is a lower hurdle than buying a card game, so it doesn't surprise me that the author didn't see many conversions. I have read that if your customer acquisitions costs are less than or equal to the money you expect to make off of them during your first year with the customer (supposing a subscription model, not unlike a card game with expansion packs), then you are doing well. The author has seen that his customers have often become repeat customers, so it may be ok to expect that $17.76 is an ok price to pay per converted user, so long as you expect some larger percentage of them will come back for expansion packs.

Some notes: DUDE! it took me WAY TOO LONG to figure out where to buy your game! The link to the product is at THE VERY BOTTOM of your Medium article. PUT IT AT THE TOP WHERE YOU MENTION IT FOR THE FIRST TIME (sorry for yelling). Make it easy for people to find and buy your game!