Disadvantages of EU Membership to UK include:
- Cost. The EU costs the UK £6.4bn. See UK government spending. The Bruges (which is an anti European group claim the cost per head of EU membership is £873, but according to Foreign and Commonwealth office the cost is about £300 per head 
- Inefficient Policies. A large percentage of EU spending goes on the Common Agricultural Policy. For many years this distorted agricultural markets by placing minimum prices on food. This lead to higher prices for consumers and encouraging over-supply. Reforms to CAP have reduced, but not eliminated this wastage.
- Problems of Euro. Membership of the EU doesn’t necessarily mean membership of the Euro. But, the EU has placed great emphasis on the Single Currency. However, it has proved to have many problems. See: Problems of Euro
- Net Migration. Free Movement of Labour has caused problems of overcrowding in UK cities. The UK’s population is set to rise to 70 million over next decade, partly due to immigration. This has pushed up house prices and led to congestion on roads.
- More bureaucracy less democracy.
Disadvantages of Expanding EU into Eastern Europe
1. Structural change as the economy adjusts to new trade patterns. E.g. many industries will not be able to compete with lower costs in the West
2. With free movement of labour and capital, the most skilled and mobile workers may go from East to West to take advantage of higher benefits and wages. This could weaken the economy
3. Agriculture may need to be reformed because the EU will not want to give large subsidies to inefficient farmers in the East. This could cause problems in rural areas.
4. The Legal and economic framework of the former command economies lag behind many EU countries however this is not an insurmountable problem.
Benefits and Costs for the EU of expansion
1. Increased Trade, This will be a small % of current GDP, but there is potential for growth and new markets in the East
2. Under current rules Eastern European would be entitled to net subsidies because they are poorer and have an inefficient farming sector. This would place costs on existing members. Alternatively they can adjust the rules
3. Increased levels of bureaucracy and more decisions would have to be made by Qualified Majority Voting