It's probably hard to do since what you're looking for is making up your own powers. Anyway.
I would use the guide from the terrain powers article to help determine what kinds of stuff you can do:
Epic Standard: Dominate, stun, or petrify
Paragon Standard or Epic Move: Blind, daze, immobilize, restrain, or weaken
Heroic Standard, Paragon Move, or Epic Minor: Grant combat advantage, allow a mark, or penalize a defense by up to –2
Heroic Move, or Paragon and Epic Minor: Avoid intervening obstacles during a move; grant concealment and/or cover; knock prone; push, pull, or slide enemy up to 4 squares; deafen; or deal level-appropriate ongoing damage
Heroic, Paragon, and Epic Minor: Add a damage type to an attack or allow a 1-square shift
I'd set base damage to high, and if you add an effect you'd slide it down by one step. So: high damage only, medium damage + minor action effect, low damage + move effect, or standard effect only. Multiple targets should probably move down one step as well.
The item wouldn't have its own attack bonus, you'd use the PC's. Attacks vs. AC should have a proficiency bonus of +2. Add in magic item attack bonuses like inherent bonuses.
The cost would be very low, no higher than 1/10th of a treasure parcel. That's about 7 GP at first level.
Here are some quick examples:
Grenade: ranged 5/10, vs. Ref, burst 1, hit: medium damage
Thunderstone: ranged 5/10, vs. Fort, burst 1, hit: deafen
Bag of Bees: ranged 5/10, vs. Fort, hit: Grant combat advantage
Firebomb: ranged 5/10, vs Ref, hit: medium fire damage + ongoing fire
Marbles: close blast 3, vs. Ref, hit: prone
Distracting Blast: ranged 5/10, vs. Ref, hit: medium damage and one ally adjacent to the target can shift 1 square