From the EasyJet website;
http://www.easyjet.com/en/Book/regulations.htm l#baggage
"Checked Baggage
Every item of standard checked ('hold') baggage will incur a fee. The applicable online fee will be displayed at the time of booking baggage (whether during or after booking your flight). Alternatively, you may choose to pay a higher charge at the airport. You will be advised of this charge at the airport. Payment of the fee provides you with an aggregate allowance of 20kg across all pieces of hold baggage which may only be increased by payment of excess weight charges.
The maximum acceptable weight for each single piece of checked-in hold baggage is 32kg. Baggage which exceeds this weight will not be accepted for carriage.
Where checked-in hold baggage exceeds 20kgs in weight (subject to the above rule), each passenger will pay an excess baggage charge per kg up to a passenger maximum of 50kgs."
I know that Ryanair charge �8 per bag if paid when booking online and �16 per bag if you just turn up with one at the airport and want to check it in.
Couldn't find the relevant cost ANYWHERE on the Easyjet site (only excess charges) - how helpful, they say you are told the cost when booking. Don't get confused between the number of bags and the weight allowance. You pay for EVERY bag, within your total weight allowance, and for EVERY kilo over the total weight allowance.