I recently did exactly what you're asking about. I bought a 3x9 on Craigslist, but never used the middle or granny gear. The front derailleur was preventing me from installing a rear fender without cutting it. I took my bike to the LBS (Harris Cyclery in Newton MA). Cost about $60, with labor. The only parts it needed were a JumpStop and a singlespeed bolt set. I'll sell the shifter, front derailleur and rings on eBay.
My commute has only modest hills, so the 1x9 setup is working well for me!

By the way, another LBS gave me a quote of $400 to convert a singlespeed to a 1x9 with good compenents.