Best is of course to simply take the Mercure Hotel Koblenz (4 star) where the conference will be. So you can meet people on the breakfast and enjoy long talks on the bar.
From there to the main station (1 km away), you find a couple of alternative hotels like the Brenner Hotel or the IBIS Hotel, both rated with 4 stars.
But you can still find good offers on the internet for hotel rooms for less than 50 Euro a day. For example the Hotel Hamm or the B&B Hotel Koblenz.
If you stay a few days more in Koblenz, we can certainly help you with trips in the surrounding land to visit some nice places.