Do prices at the deadline actually go up or by that much? Or does it just depend on the year?
Honestly? I don't think so. I mean it's all a gamble. Prices start really high, simply because the sellers have time on their hands. Right now Botts is asking for a 1st++ for Kane. Why? Because he has between now and the TD, and isn't under any pressure to sell now, and thus is free to ask for a sky high price in hope that some desperate GM will pay it. Where as the team buying is (in theory) under pressure to acquire that high end talent asap to try and win more games. However the closer you get to the TD... Botts loses the advantage that he has now (aka time), and his only play then is to play GMs off against one another. If he has multiple bidders bidding, and bidding seriously, he can use that to keep the price high. But I think the ask for Kane today will be less then what Botts receives when Kane is finally traded.
But then that's all dependent on supply and demand. Remember when Cammy was a Flame and Burke couldn't even get a 2nd for him? So instead of selling him cheap (I think the best offer he said he'd received was a 3rd+) he said FU to everyone and kept him. That year there were a bunch of quality forwards on the rental market, which drove down the prices for everyone. Anyone who sold before the TD would have received better prices then those who sold at/near the TD.
Bottom line? Prices can be all over the place. They'll start high, but there's zero guarantee that they'll stay there. And on the flip side... there's zero guarantee that they'll come down. It changes every year based on supply and demand and how desperate the buying GMs are.