Bandwidth costs money. The Switch from Nintendo offers “First Taste Is Free” online play. Ok.
But a 300 buck system (if you go buy PS4 standards, like the stolen snapshot button) would likely have 100 dollars a year of play you may not play may be more likely victim of DoS attacks by prank hackers.
When if your playing offline games for months?
Honor system may work well, plead that it costs to run servers and all that….ask for 5 dollar donations or more, offering big rewards for big helpers (like with crowd funding). Or a huge cost deduction in the hardware with a standard subscription fee for online services not used to sell downloads.
But frankly your likely to be in a huge financial hole, with the Switch, and think of interest on your card.