The cost of living is generally on par with the UK, outside London. Rent is often the greatest expense. A one-bedroom apartment might cost £800-£1000 per month depending on location. Many people moving to the islands choose to house-share, both to reduce their rent and share living costs, and to make new friends.
The top rate of income tax is 20% and social security is 6%. There are no capital taxes such as inheritance tax or capital gains tax. While neither Jersey nor Guernsey has UK VAT, Jersey currently applies a Goods and Services Tax of 5% on almost everything, including food.