Bizrate, Nextag, Pricegrabber, Shopping & etc.. Which Offers Best in ROI?

Hello,
Let me start by introducing myself. I have been currently developing a eCommerce store dedicated to selling aftermarket automotive parts (USA). The site is almost ready for launch, and I need to start figuring out a marketing plan.
It would be extremely helpful to get insight from users on this message board that has used one of these 3rd party shopping search engines.

How PPC campaign gave the best ROI?
How much you spent on your marketing campaign monthly?
Would you recommend using this 3rd party search engine? Why or why not?

Thank you in advance for helping me on choosing the right marketing path.

See original post by philee