How much will it cost ?

There are two elements to this. Court fees and Solicitors costs.

(a) Court fees are set by the Courts and are based upon the value of the claim.
(b) The Solicitors costs wil depend upon how the person who owes you the money responds to the claim.

The costs will rise if they dispute liability and lengthy negotiations are involved. However, you can purchase costs insurance in the event of you losing your case at trial.