There really isn't a formula... or rather, it is quite lengthy and involves things like the number of people who voted that day.

There are some websites that calculate vote amount.
But, generally, at 488 SP, at todays current prices, the upvote is worth about 10¢ at 100%/100%. 100% voting power and 100% vote allocation. (at 488SP you get to have a slider that allows you to change your vote %)

The "Sell" function under STEEM is ... mislabeled, it would be more appropriate to say "Send to someone else".

Steemit has an internal market where you can trade STEEM and SBD, but that is it.
If you want to trade STEEM of SBD for bitcoin, litecoin, BitShares and such, you can either transfer your steem to an exchange like Bittrex or you can use Blocktrades.us to transform them (it trades them for you, and deposits the outcome in your wallet).
There are many articles about this on steemit.