Varies by material and labor. Removing and replacing the dash isn't hard, but time consuming. Recovering the dash takes some skill, but many do it themselves fine. Vinyl is fairly cheap, Porsche grade leather and automotive grade Alcantara are not cheap. Ballpark $300 or less all cheap and all DIY, to maybe $1500 for a good shop and materials.
Rob recovers your dash starting at $700 plus shipping.
Porsche 928 Dash