Do you know about steemwhales.com? If not, take a look at that site.
Will show you details of any users wallet.
You can look at my wallet and see how SP and STEEM varied over last 4 months.
I suggest looking at jerrybanfield wallet for a cleaner picture.
Jerry estimate account value was $130K on October 1st
By Oct 30, Jerry estimated account value was $85K at end October.
Major loss of value over month of October.
SP value relative to US fiat, goes up and down the same as STEEM value relative to fiat.
If the price of STEEM is diving relative to US fiat, SP is doing the same. More or less. Not exactly, but close enough to be approximately correct.
STEEM is currently $0.90. If you think STEEM is going up, buy and wait for STEEM price to increase.
Putting your STEEM into SP does a couple of things:
a) locks you money up, and to get 100% back you have to go through a 13 week power down cycle. So your money is locked for 3+ months
b) in exchange for having your funds locked in SP for 3+ months, the payouts you get for posts and replies/comments will be adjusted slightly upwards, and the value of your vote when you comment on other posts will go up.