Soft jaws should only be about £20-£30 a set depending on the chuck manufacturer.

They do have more uses than just boring out the inside diameter to prevent runout, you can also bore out the face's with a shoulder for holding larger dia items. Say you had a 100mm dia circular bearing holder with a finish width of 20mm then you could bore the jaws at 100mm dia down to a depth of say 15mm, cut the 100 mm dia material a couple of mm's oversize in a saw and then when you put it in the chuck and tap it up to the shoulder it will run true and will be easy to get the width parallel.

Also if you bore them to the size of the part you are machining then it helps prevent marking of the workpiece that hard jaws can sometimes leave, specially on soft material like aluminum.