Softwood doors with a hardwood veneer can be some of the cheapest, with UPVC being a close second. Aluminium and composites will cost more. UPVC is a good choice because it’s relatively inexpensive, a good insulator and lasts a long time.
If you are seriously considering fitting a new set of patio doors, you should check out this article: https://www.doubleglazingpriceguide.org.uk/how-much-do-upvc-patio-doors-cost/