what is a strong fabric for furniture upholstery, but with the lowest cost

Grumpelstiltskin

OP If you are not used to doing upholstery then my advice is don't bother. How are you going to fix the fabric?

If it's stiff it's no use trying to sew it with a normal sewing needle and normal sewing thread. that isn't going to work, if the frame is metal you can't use tacks to fit it and any sewing needs to be able to take the weight of an adult, so not easy.

button_box

I think we need a bit more detail about the construction of the chair you are hoping to cover. If the seat base is wooden and secured to the metal frame it would be possible to staple a cover to the seat. If the seat base is metal you are going to have more of a problem and you will need spray glue to attach the replacement foam pad to avoid it shifting when anyone sits on it.  £8 a metre is quite inexpensive for upholstery fabric. I think others suggestions of checking out charity shops and fabric shops for remnants is the way to go if you want something cheaper. If you have a Repair Cafe near you then someone may be able to help you or advise you. They maybe even have suitable fabric remnants. If the chair is not used on a regular basis how about removing the current upholstery and painting the seat.