Is there an extra charge for Wifi Extenders

If you lease the AirTies extenders from AT&T it's a $10/mo fee (you can get as many as 3 units for that fee).  That being said, I'm not a fan of the AirTies as they have some significant issues with both usage and reliability.  I don't know of any other third party extenders that don't work so you have a ton of choices there.  However, you might want to look at mesh wifi systems which do a better job if you need wifi over a decent size home.