ABS sensor fault

[Tom360]

Going to look at a clio 172 on friday but he told me the ABS light comes on after about half a mile and need to buy a ABS sensor. I asked him which one i need or tried to find more information about it and he does not know. He just said it will cost 25quid for part.

Is this costly? In labour or anyone else had same problem.
Any advise would be great thank you.

 

[MicKPM]

Unlikely to be the sensor but another fault as its passing the post checks. I'd guess a wheel target is damaged/cracked

 

[Tom360]

If it was the wheel target that is damaged/cracked what am i looking for as i dont have a clue what that is?
How do i find out?
Just say the ABS lights stays on from starting the engine that mean its the sensor or the brake caliper is at fault?
Or if it only comes on after half a mile then it could be wheel target?
Sorry to sound dumb, Thank you for reply.