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you can look at the price feed on steemd when you load the post then again when you vote

This web site shows the current dollar value of Steem: http://www.steemdollar.com/steem.php

And here you can get an excellent chart over time of Steem vs. Bitcoin price (which you can infer dollar price from if you really want to): https://poloniex.com/exchange#btc_steem

Basically Steem has been in a gradual downtrend since late July.