While I do agree that it could make things easier for traders, I also believe it is not really needed and it wouldn't be as good as it may seem.
As @iGuinho pointed out, prices change daily and that's something this power wouldn't take into account. It would only compare the amount you paid for certain powers with the amount you entered and give you a feedback, making it an inaccurate tool as it could wrongly classify a trade as positive/negative when perhaps it isn't if you consider the current prices.
Sure, you could also check the FairTrade Guide/use a bot to make sure the price is actually reasonable, but wouldn't that make this power kinda useless then? You'd still be pretty reliant on the current price to make a decision.
The only scenario where I could see it being useful is if a user doesn't necessarily need or want to sell a power but they check the price after a while and they are curious to know if they could make a profit.
Maybe at a lower price (400-500 xats), it would be okay.