OK so what are you talking about specifically here? You talking about picking up Wagner's option and trading him? because that's a possibility. As long as he wound up where he could close I doubt Wagner would mind that much.
As to your actual question I'd say it depends on a ton of factors. The age of the closer, the state of the market, the state of the league, how in vogue the "position" of closer is among baseball minds at the time, and perhaps most importantly, how much potential buyer teams believe in their current idea of who should be their closer. Right now I'd say that a good closer type won't fetch top dollar because most of the teams that need a good closer have one and most teams that don't aren't the ones that believe in trading for well-remunerated veterans