I think each player should get 1/2 a tick of xp and gold (multiplier applicable) for every kill. They should get between 1/10 and 1/4 of a tick for assists (depending on damage done). There should also be a damage counter and at the end of a map (time runs out, score limit reached or new map poll accepted) the player should get xp and gold (in a ratio relative to the tick xp/gold ratio) depending on their total damage done. Every kill should add 0.1 to multiplier and every death should remove 0.2 from the multiplier.
Also, on a side note there should be a marketplace function to offer x amount of gold for x item, rather than having to accept someone's offer of x item for x gold.