Somewhere around £30 at screwfix or toolstation.
A Direct On Line (DOL) motor starter includes a "thermal overload" relay, sometimes included when you buy it sometimes not. Either way, they are often the wrong rating so you may well have to pay an extra tenner at the same time and buy one that fits the starter and is the correct rating for your motor.
When they are included, they are usually on the high side - for bigger motors like 2-3 kW. These can't be adjusted down far enough for smaller motors.
To get the correct overload, look on the motor nameplate for the current rating in Amps (this is the Full Load Current). Make sure that this falls within the range of the thermal overload that you buy. They are adjustable between a max and min current. When you connect it up, adjust the overload relay to match the motor FLC. If you don't either it will leave your motor to overheat when you need it to trip, or it will trip out too often.
You would be amazed how many people buy a starter and can't be bothered or don't understand the need to have a properly sized and adjusted overload relay. It's an insurance policy. £10 now to stop you accidentally burning out a £100+ motor.
I'm going to pass on the issue of wiring it in. They are a little complicated if you have no experience so my advice is get someone who knows what they are doing to help. This thread just lets you know what the real cost of the replacement parts