we are up in the midlands, generally the cost of making a new box sash is around 1000 plus VAT, if they are in a bad state it can cost nearly as much to restore them, we have done some in the past that really needed new ones but the customers have wanted them restoring. Once restored and in working order they are great windows. The main downfall of them is people just keep painting them on top of the old paint instead of burning them off. Then they end up painted shut, the paint gets that thick and flakes off or cracks and the water get under it, then it gets the sun on it and they start at rott.
if burnt back and repainted when needed they should last 100 years