It sounds a bit high to me, but without knowing exactly what they did it's hard to say. From the sound of it they probably charge by the hour and spent some time working on your camera. It should be easy enough to find out.
Sure you can continue to send your unit in for cleaning every time it gets dirty, but it seems like it will get very expensive. As mentioned above, there are much cheaper and very safe ways to have a clean sensor.
I use the Fuji S2
Pro and when my sensor first needed cleaning I followed the manufacturers guide-lines for cleaning. I purchased Sensor Swabs and Eclipse from http://www.photosol.com
The sensor was like new afterwards.
The problem of the high cost of cleaning sensors has been addressed on the forums of http://www.dpreview.com
I found the following link there http://www.pbase.com/copperhill/ccd_cleaning
,which shows you how to make a inexpensive cleaning tool which some photographers claim works as well or better than the recommended Sensor Swabs. You will still need to get some Eclipse and Pec Pads.
I would do a little reading up from these sources and try cleaning the sensor yourself next time.