Western saddle? I had Amish do mine in fake sheepskin for about $50. The real sheepskin was $100. They didn't do the greatest jo. Just took the skirts off, stitched new fleece onto them with a machine, and didn't try to reuse old stitching holes or anything to do a really super-neat job of it.
All that fleece really does, afaik
, is keeps the saddle pad from skidding out from under the saddle. So, even rough leather there will do the same thing. If it's a really good show saddle or something of sentimental value, I'd pay to have it done right. But otherwise, it's not worth spending on.
English saddle? If the leather's not cracked and it had wool flocking, it can be restuffed for $100 - $200. If it's serge (fabric) or cracked leather or leather covered foam pads that broke down, then replace the whole panel with new leather and flocking for $265 - $300 is required.