Alpaca is a cheaper alternative to cashmere, and is very warm.
Can you knit? Knitting your own sweaters is not a cheaper option, but it does mean that you can make ones that are right for you (as regards not too tight round the neck etc). With careful shopping for yarn, you can make sweaters that are much better made and of much better yarn than shop-bought ones. I use Colourmart.com, which sells 'mill ends' of cashmere (the bits at the end of the massive cones that are used commercially) at good prices. You don't need to buy huge amounts - the smallest cones are usually 150 gramma nd three will usually be enough to make a jumper.
A good quality jumper (bought or hand made) won't go out of shape when washed. Acrylic might, as it has little 'memory' (the ability to spring back into shape). Natural fibres can be reshaped when damp and will hold that shape when dry.