you should focus on your main site, if you make a forum, it will make your work increase, maintenance and monitoring forum not easy...lot of spammer will come there

Adding a forum can either add to or detract from your site. It really depends on what the intended purpose of your site is. If you just want people to go there to buy a product or service and nothing more, then it is probably not worth the effort. If you have low amounts of traffic to the site, then its probably also not worth it. However, if you have or intend to generate a large amount of traffic and return visits to your site, an active forum can really help with that. Forums will typically work best for niche sites with a very passionate audience, as they are most likely to want to discuss topics within that niche.

In short, your site primarily focuses on comparing prices, and while many people are avid bargain hunters, I can't see a forum really benefiting your site that much. The choice is up to you though.