I'll try to explain it simply.
If you have 150 steem power and power it down you get 150 steem.
If steem is worth $2 each when you start powering down and worth $1 each when you finish powering down, it's irrelevant, you powered down 150 steem. Prices change, that's all crypto and currencies in general. Just because you start a power down when steem is worth $2 each doesn't mean the rest of the world won't continue to change the value of steem while you are powering down. We all continue to do our transactions and you still have your same amount of steem. If steem had become more valuable while you powered down your total would have increased. It's a simple supply and demand economy.