The savings are so immense depending on where you get your grain that you'll pay for the mill in just a few sacks of grain. I just got a couple sacks of 2 row from a group buy here for ~$75? 68 cents/lb versus the $1.75 or so my LHBS charges adds up fast. I still buy a LOT of stuff from them including specialty grains and such, but I use too much base malt to be paying such a premium.
I wouldn't buy a full sack without a mill to cut to the point. It would be an interesting experiment however. I roast my own coffee as well and I can taste the difference between coffee that was ground the day before and fresh ground. Maybe I'll crush a pound of barley and make barley tea once a week and sample it against fresh crushed barley tea to see when I can tell oxidation has set in...