Dragohide full plate costs twice what masterwork full plate would cost, or 1500 gp + 150 gp for masterwork = 1650 gp; so 3300 gp for dragonhide full plate. Making it a +2 suit of armor is another 4000 gp, so 7300 gp.
The armor itself will be immune to that dragon's energy type, but it doesn't automatically imbue any resistance to you. You do however get a 25% reduction in the enchantment price for any Energy Resistance. You can add 10 points of any energy resistance to a suit of armor for 18000 gp to its cost, 20 points of resistance for 42000 gp, or 30 points of resistance for 66000 gp.
If you wanted to make that +2 silver dragonhide full plate armor also grant Energy Resistance (cold), that's where the discount would come into play. For Resist cold 10, it would be 7300 + 75% of 18000, or total 20800 gp. Similarly, you'd have to spend 7300 + 75% of 42000 gp, or 38800 gp total, to get Resist (cold) 20. Resist (cold) 30 would cost 7300 + 75% of 66000 gp, or 56800 gp total.