> the conversion cost of 1 Steem power via 100% power up vs converting Steem dollars?

It fluctuates and I'd struggle to put it in such a relation, but I'll try to give you a long-winded reply on how the rewards work instead :P For making it easier to grasp, I will invent some unit names (in reality they are RShares and Vests and whatnot and the numbers are a tad confusing).

Your content is rewarded a portion of the available reward pool based on the amount/strength of votes (RShares) it collects. This total value is shown on your posts as $xx.xx.

Now this $-Sign actually kind of stands for SteemDollars treated as US Dollars - Let's call them RewardDollars for now.

At the time of payout these Reward Dollars will be credited in either 100% SP (powered up STEEM) or 50/50.

This conversion from "Reward Dollars" to STEEM/SBD happens on the assumption that 1 SBD was worth exactly 1 USD and converts according to the STEEM-USD price known on the network.

The STEEM-USD price on the network is a moving 7 day average ($US 3.65 atm).

As an example for a projected $10.00 reward and a STEEM-USD rate of $4 and a SBD-USD rate of say $3 (AND ignoring curation rewards):

100% = 10/4 = 2.5 SP = $10 USD
==> 2.5SP / $10 USD

50% = 5/4 = 1.25 SP = $5 USD
50% = 5 SBD = $15 USD = 3.75 SP
==> 5SP / $20 USD

Now this is not really intended behaviour an probably a temporary situation, the USD peg in the SBD design really only works to keep SBD at least on USD level but does nothing to keep it from exceeding. Witnesses are discussing possible solutions and IF this should be "repaired" these days.

But for now... as long as either STEEM or SBD are above 1USD, you should always chose 50/50... as you can see it can make quite a difference!

*P.S.: oh and... welcome to steemit :D*