you can clean the sensor yourself... the mirror too if you're careful (it's a front surface mirror). cleaning the viewfinder is tricky, dust and dirt will come and go. have a go at it with some dust off... careful to "clear" the nozzle first, and keep the can vertical, especially if it's a new can. it's easy to clean the outside of the viewfinder... q tip with some lens cleaning fluid is the tool.
a repair service will charge a lot for this work. to do a comprehensive cleaning of the viewfinder would require disassembly of the camera... not something you want to get into and WAY more expensive than any advantage you might gain... especially as it doesn't affect the photos, only your view. Unless the viewfinder system is so dirty that the exposure system or focusing system is not working, it's best to give the outside a quick wipe and that's it.
For sensor cleaning read this:
Cleaning sensors by Thom Hogan
best gadget for sensor cleaning:
sensor swab... (pick the right one for your camera model)
Cleaning Digital Sensors, Cleaning Digital Cameras - Photographic Solutions, Inc. - Digital & Photographic Cleaning Solutions
I know people get nervous about this, but trust me, it's not rocket science. just take your time until you get the hang of it. might take several attempts the first go-round, but it's a good thing to know how to do.