Is my local bike shop asking to much money to change a rear cog?
Ah the internet.

Answered my own Q:

It takes about a minute. Yay!

Watch at 2:13: Replace Fixed Cog (broken link)

Instead, try


How To Change A Track Bike Or Fixie Sprocket  (splined).
FIXIE COG INSTALL #3 (threaded)


For DIY: need a chainwhip and a lockring wrench / lockring spanner 

...or a multi-tool with chainwhip &amp; lockring wrench!

Done.