For manual wheelchairs, you can buy spare types for both the large back wheels and also the front castor wheels, they usually come in sizes between 18 and 24 inches for the large back wheels and there usually isn’t much price difference between the small castor tyres and the large back tyres but fortunately they are generally inexpensive.
Because the tyres are taking the full weight of the wheelchair and user, they will in time wear down and less grip on the tyres can mean less traction and require more of an effort by the wheelchair user, but there’s also the factor of safety, for example making your way up or down a ramp, you want to make sure that the tyres have enough grip on them to prevent any chance of sliding or not being able to stop properly.
Both the castor type and the larger tyres will usually be priced at around £8 upwards, generally costing not much more, and thats cost per tyre.
When buying spares and replacing parts on a manual wheelchair, the tyres are probably one of the most important to deal with as they are the parts touching the ground and taking most of the wear and tear over time.