Average Cost of Wheelchair Tyres?

Hi,

I'm hoping somebody can give me an estimate, as until now I've never had to replace them but these have had their day. The back tyres are bald & the front ones are heading the same way.

I use an electric Salsa Quickie, which I'm totally dependant on for independence. The back wheels are solid & roughly the same size as those on a car. The front ones are solid &rougly the same size as those on a standard electric wheelchair.

Sorry about the vague description, but I'm no expert.

Thanks in advance for any help you can offer.