This really depends on the part. But EVERYTHING has gone up cost wise for our older machines and I don't think it is going to stop anytime soon. I used to save it all, but it is just not possible. So save wisely. Basically, I save any part that I think is rebuildable (even if I don't want to take the effort to rebuild it myself). If it is rebuildable, it has value and will always have value to someone. Speaking of the dash, I sold a complete one (gauges and all) for $250 about 6 months ago. I didn't need it, but yes, they are getting harder to find, but $1200 is insane and even $600 would be too high for me. In cases like this, you can just put out a wanted ad on various boards and usually one will turn up, or just wait longer to find one on ebay. But remember, a part that you need "NOW" is always going to cost you more than one that you can wait for.
I save all bodywork too, even bent wing panels, because they're just getting too hard to find now. Some people used to save old wheel cylinders, and those have always been throwaways to me, but a lot of the new parts (brake parts specifically) are such low quality that people are having their old wheel cylinders refurbed with stainless liners.
Used seats aren't worth too much (to me). I put the Exmoor ones in mine. I have a few old ones laying around. Some people have them recovered if the bases are in good shape. But I imagine the cost on that would be as high as replacement seats, unless you do that kind of work yourself.
61 II 109" Pickup (Restomod, 350 small block, TR4050)
66 IIA 88" Station Wagon (sold)
66 IIA 109" Pickup (Restomod, 5MGE, R380)
67 IIA 109" NADA Wagon (sold)
88, 2.5TD 110 RHD non-hicap pickup
-I used to know everything there was to know about Land Rovers; then I joined the RN Bulletin Board.