A report by Open Europe argues that for the UK the cost of Brexit would be minor. The benefits might be minor as well. For interest groups could make it hard to reap the potential benefits of newly gained flexibility.
… the path to prosperity outside the EU lies through: free trade and opening up to low cost competition, maintaining relatively high immigration (albeit with a different mix of skills), and pushing through deregulation and economic reforms in areas where the UK has historically been sub-par compared to international partners. … whether there is appetite for such changes in the UK is unclear.
… implications for the type of relationship the UK should seek with the EU post-Brexit. Realising the potential economic gains we’ve identified – notably via immigration and deregulation – means a relatively high degree of flexibility from the EU. The confines of a Norwegian or Swiss-style arrangement would not deliver this. As such, the best option would be for the UK to pursue a comprehensive bilateral free trade agreement, aimed at maintaining as much of the current market access as possible while also adopting a broader liberalisation agenda over the longer term.
Update: The Economist reports about other cost/benefit estimates.