If you have some kind of a spreadsheet program (such as Excel), it is pretty easy to keep track of your average cost per BTC at various points in time, and surely having a spreadsheet, there are all kinds of things that you can keep track of.
You can make your own Excel spreadsheet for tracking your investment portfolio, entry prices, average investment price, cost and profit or loss with each entry and with your whole portfolio.
If you don't want to make your own Excel spreadsheet, you can use available websites like this
https://costavg.com/It has many helpful features and can save your time.