The first thing to do is to research the price of similar properties.
Fortunately it is now possible to get a listing of house purchase prices - you just have to find a similar property.
A number of web sites offer this service:
www.myhouseprice.com
Daily Mail readers can use this service free of charge at the moment via www.myhouseprice.com/dailymail
www.nethouseprices.com

A web site that will provide a valuation is
www.mypropertyvalue.co.uk

Have a look at web sites that give properties on offer
Before placing a property on the market get a valuation from a number of estate agents
Some of these should tell you the prices of similar properties that have been sold
The price they give you will vary depending on a number of factors
They may price the property high to pursuade you that their marketing enables them to sell at a premium and to place your property in their hands.
They may price the propery low because they want to make a quick sale - tell them you are seeing other agents and this should avoid this problem.
They should give two prices - a price that you should expect to make and a price that should generate interest.
Remember the price you market the property at is unlikely to be the price you will be offered or sell at.
At the moment you will be offered less than your stated price and you would expect to get more than the first offer.

Costs

Solicitors (conveyancing) fees - typically £300 - £600 depending on house price
Estate Agents fees - typically 1 - 2% but negotiable.
Stamp duty on purchases - 1% £60,000 - £249,999, 3% £250,000 - £499,999, 4% £500,000+ www.inlandrevenue.gov.uk/so/so1.htm
Note - selling at £500,000 will attract £20,000 tax, £499,999 will attract £15,000 tax the jump should be to appreciably more than £505,000.
Mortgage fees on purchases
Mortgage early repayment fees on sales

www.www.home.co.uk