My question would be, how can you compete with the big boys for traffic? At the end of the day I would suggest you need good content and an established site. Both of which take time? The major comparison sites will allow affiliates access to their feeds (afaik) so setting it up is easy? But you need some plan to get the traffic! You have to provide some "value" over and above what is already out there else you are never going to get anywhere? How would you plan to do that? If you have a good idea - go for it
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One thing I have learnt is that, unless you get lucky (ie with an original idea) then it takes time to build a network of sites that can start to make some decent money. If it was easy, we'd all be retiring by now
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I think your level of commitment is also key, are you looking at setting up a full time business and hiring some people (new comparison sites are doing this even now) or just trying to make some extra cash in your spare time. Personally Id think its got to be very hard to compete with the former if you are the latter? These sites have dedicated teams negotiating with suppliers, dedicated techies, sales teams,
SEO specialists, etc. The peeps who seem to do really well out of this approach it from a full time business point of view. The "get rich quick" approach tends to yield some pocket money but, unless you get lucky, it will take a long time to make progress. But a journey of 1000 miles starts with a single step
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Try piggy backing off one of the major sites. Or, as we've all heard a million times, try to find an under exploited niche!