The upgraded set's bonuses are backwards to the non-upgraded set pieces so you don't lose bonuses when you switch every 2 pieces out. Not sure if you can wear 2 pieces of each of the caster sets giving you like +600 armor. That's an interesting thought.
Also, you can only upgrade the set that was intended for your class so a priest can't upgrade deathmist and so on.
As for the total cost, the TEST server gave us about 250 gold, but it already gave us the quest mats we needed so it'll cost way more on LIVE servers.
Here's a decent walkthrough I found.
http://www.wowwiki.com/Tier_0.5
Also, you can only upgrade the set that was intended for your class so a priest can't upgrade deathmist and so on.
As for the total cost, the TEST server gave us about 250 gold, but it already gave us the quest mats we needed so it'll cost way more on LIVE servers.
Here's a decent walkthrough I found.
http://www.wowwiki.com/Tier_0.5