You're conveniently not factoring in all the benefits of being an EU member brings to the economy.
I'm really tired of people trying to spin that EU membership is some sort of financial drain on the country.
Besides immigration the only reason I can think of for them wanting out is the unwillingness to further integrate into the EU. If Britain it can do better on its own then by all means, have at it. Just don't expect to have the same bargaining power as before. The illusion that you'll have more freedom to trade with other countries will fall flat on its face once you realise that:
1) The EU will still be your biggest trade partner whether you like it or not, only now you'll have no say in any new laws they plan on passing. Your goods will also be less competitive in price compared to other EU countries.
2) The US, Canada, Japan and other countries have stated before that they will not only find the UK a less attractive trade partner if it leaves the EU, but they will also move their business elsewhere.
A Brexit will negatively impact the economy, that is a FACT... not to mention the social and political ramification of such a move. The real question is if the "freedom" you get in return will be worth the cost.