Can I install a backflow preventer for a frostless hydrant on well?
I will potentially be drinking the same water that flows back in the well after the frostless hydrant is shut off - yes?


No.  Freezeless faucets drain their water back into the ground at their base below the frost line when turned off, NOT IN TO THE SUPPLY.  The freezeless facuet drains this water so the pipes above the frost line have no water to freeze in them; hence a freezeless faucet.


  Will a simple $5 plastic backflow preventer be all I need?


You get what you pay for.  Will a 5$ part function as well as a 125$?  Of course not!  Will it work well out of the box? probably yes and probably fairly well.  But it will not be as effective at stopping backflow, it will probably not work for as many years to come as a true backflow preventer, it probably cannot be tested, and it probably cannot be repaired.


  ...or is there a certain product meant just for this?


There are true backflow preveters for this that are designed to effectively stop backflor that are able to be checked and tested for reliability and repaired.  These typically installed run 200-300 in parts alone for the backflow, fittings, boxes, etc.  Labor can easily double or triple this in many cases.