If Internet retailing were a board game, the goal would be labeled "ROI," and comparison ­shopping engines would be one way ­merchants could try to get there. Like any route to a game`s end, it`s a path with potential opportunity: It can attract highly targeted shoppers that produce what some say are higher conversion rates. And it`s got obstacles: the difficulty for some retailers of wrangling product feeds into the engines and the fact that costs per click, as in other web-based ad programs, are going up.