Flying out of Frankfurt

Since your flight is not first thing in the morning, you might want to look into staying at nearby cities that could have direct train the the airport. Google Maps says that Mainz and Wiesbaden are close by.

Where are you coming from? You can get right to Frankfurt airport from just about anywhere within a 100 miles by direct train in an hour or two.

There is a big trade fair that week, so this drives hotel prices really high. The Tourist Info often has blocks of rooms at a lower price though, so check with them. Near by towns like Mainz, Wiesbaden, Bad Homburg, or Kronberg may also have high hotel prices. Eltville might be a good possibility.
http://www.frankfurt-tourismus.de/en/frankfurt/ukv?reset=1

Or check this hotel:
http://www.kleine-villa-frankfurt.de/index.html

We have stayed in Mainz the night before a flight. We have also stayed at the beginning of a trip and spent a day in Mainz recovering from jet lag. It is a lovely and relaxing town. We stayed at the Hyatt and at the Hilton (there are two and we chose this one: http://www3.hilton.com/en/hotels/rhineland-palatinate/hilton-mainz-QMZHITW/index.html). Both of those hotels are on the river, and while they may cost the same as the Frankfurt rooms their scenic locations are worth it.
Mainz has a pleasant pedestrian plaza with shops for those last-minute souvenirs, and there are plenty of places to eat as well. It is a short train ride (maybe 30 minutes) on the S-Bahn, and there is a stop at the airport. Arrive early in the day and you can sightsee a bit, including the Gutenberg Museum. The hotels by the river are a bit of a walk from the train station, but it is an easy walk/short cab ride and I think the beauty of being on the water makes it more memorable.
Enjoy.

You said you need to be at the airport around noon. I assume your flight leaves arount 2:30-3:00 pm and that leaves enough time for check in, etc. I've had flights out at similar times and spent the night once in the Black Forest outside Karlsruhe and came in on an ICE, and once in Boppard (on the Rhein, a little farther away then Bacharach) and came in on an RE with no problem.

Depending on which direction you are coming to the airport, you might want to stay in Mainz or on the Rhein.

The first place I would try would be Hotel Königshof in Mainz. It's a comfy traditional hotel with a great breakfast buffet included, with a short walk across the Bahnhofplatz to the Mainz Hauptbahnhof, from which there is a 26 minute S-Bahn to the airport Regionalbahnhof every half hour, as well occasionally as some faster regional trains. Looks like they have double rooms available for the date for 108€.

If not there, there is a direct RE leaving Boppard at 9:15, stopping at Bacharach at 9:35, and getting to the airport Regionalbahnhof at 10:34. There is also an MRB leaving Boppard at 10:07, St. Goar at 10:19, and Bacharach at 10:30. It gets to Mainz around 10:30, where you have a 7 minute, cross-platform change to an S-Bahn, which gets to the Regionalbahnhof at 11:59. There are plenty of less expensive accommodation choices between Boppard, St. Goar, and Bacharach.

Digging into this a bit on bahn.de, looks like there is a direct train from Mannheim to the Frankfurt airport, and it's only half an hour with no changes. Hotels look pretty reasonable there on booking.com. Another possibility is to look at VRBO. You might have a hard time finding someone to take you for one night or incur a cleaning fee that might make it less economic to go that route, but it's worth considering. People renting out their own places are probably less aware of the trade show schedule or might just not get around to changing their listing to take advantage of the heightened demand.

Hi Hud- and if you have a car to return you can drive to Frankfurt from MANY towns. I tend to favor Heidelberg or Karlsruhe. Before the days of increased security I once drove from Munich and, after a brief stop on the side of the road to pay a speeding ticket, caught a 10:30 AM flight with time to spare ...

Have a great trip!!!

Thank you for all the response. . . my original plan was to travel from Munich to Frankfurt the day before the flight, until I found out about the price hikes

You can leave Munich at 7:28 and be at the airport at 11:06 on a direct train, and still have two back up trains that arrive at 11:22 and 12:06. You could also spend the night in Nuernberg or Wuerzburg for extra safety factors.