Hi Roz -
how long do you need the room(s) and what counts as too expensive? There is the ACHAT Hotel just at the start of Griesheim. The tram goes past the door so it's an easy trip to the station and other parts of Darmstadt. Invest in a week-long tram/bus ticket and you can visit most of Darmstadt easily enough. The Achat is nothing very exciting but it used to be OK priced and some of the longer term rooms have cooking facilities. Otherwise the IBIS hotel in Darmstadt used to be one of the cheapest - close to the centre and not too far from the station. Again, the tram isn't far away. A quick look online suggests these are around 55- 65 Euro per night per person in a double room... but I didn't look hard for better online prices. They're probably lower over weekends when the conferences aren't on.
Another one in Darmstadt that used to be a favourite for guys starting work where I am (and maybe still is) was the Weinmichel (fuill name is Add on Hotel Weinmichel I think). They did cheaper prices for longer term stays.
Darmstadt is a pretty expensive part of Germany so the hotel costs are quite high. I haven't really heard of any B&Bs in the area. It's more a business destination than a tourist one.
Here's a link to Griesheim hotels as there are some cheaper options there:
http://www.griesheim.de/Hotels.756.0.htmlGood luck!~