Check out surplusrifle.com, they've always got good info for starters.
From what I've read here recently, the No1 Mk3 was going for around $200- 250 (someone help me if I'm wrong). I got mine about ten or eleven years ago for $125.
Unfortunately, the surplus ammo has dried up, so factory loads cost anywhere from $16-$30 for a box of 20 rounds. You'll want to check headspacing as some bolt heads allow for different ammo.
It's a joy to shoot though and the action is fast, I love mine and look forward to getting a No4 Mk1.