You will find lots of Mosin addicts and experts on this site. These were for the most part fairly easy to find until recently. They are still available, but getting harder to find as the surplus is being bought up. It seems to me that these have taken on a sort of "cult" type following and once you own one....you will own two, then three, and so on and so forth. They are an aboslute blast to shoot (no pun intended.) It is by far one of my favorites to take out. They are loud and obnoxious, and the M44's have a fireball that projects from the end of the barrel when shooting. I don't know how else you would classify a good time!
The going rate for an M44 is about $100 to $140. With $100 being a decent deal, $120 about normal, and $140 being an outstanding rifle or someone is charging too much.
The 7.62x54R ammo is fairly easy to find, and fairly inexpensive as it relates to other rifle ammo. From AIM or Sportsmansguide and other retailers you can find 440 rounds for about $90, and most surplus ammo is about the same, approximately 400-500 rounds for $100-$120. Now you can certainly buy much more expensive ammo for it, but this is the cheapest. Also, much of the ammo for the Mosin's is corrosive, so make sure you clean it properly after use with some windex or water.
Also the site below has a TON of great info and everything you would want to know about the Mosin's.
7.62x54r.net