Some charge a large setup fee upwards of 1,000. Others charge flat PPC but require 100-250.00 deposit and some allow payments as low as 25-50 to get started. Varies greatly between niche/markets as well as companies you choose to market with.
Lets just say for the price, I would stick to PPC. Comparison Shopping Sites are stuck in technology that is 10+ years old. Top Tier PPC sites have spoiled my idea of service and i've tried the big CSN's and none of them offer any "technological" solution to getting legitimate hits - you just get a flood of traffic.
Comparison shopping sites operate on a cost per click model where your products show up based on price you are selling it for on your site. If you are the lowest prices on the shopping site you are likely to get the visitor and sales. Most companies I know that use these sites do quite well with them.