Auto repair question for experienced mechanics - alternator -?

I have a fairly old '86 chevy with the old 350 V8 used for so many years. Its got almost 140k on it but has been running fine until yesterday. This car also has some of the digital gauge readouts GM was playing around with in the mid 80s on the instrument panel. Yesterday the Volt gauge lit up with a red warning light next to it, and it is reading a charge output (I believe its in Volts) of approximately 11.3 to 11.7 depending on speed and accessories turned on. I am by no means an experienced expert on charging problems - but I 'think' a normal charge output reading would be somewhere around 13 to 14 volts, in that range, wouldn't it? My question is, whats the most likely problem causing this - is it usually always the alternator needs replacing (I assume most alternators even in the mid 1980s had the voltage regulator built into it (?) Or are there often other issues that can cause a sudden drop in the voltage charge output ? I assume the mechanic can hook up a gauge to the alternator and measure its actual output - If it does need to be replaced, what would be a reasonable price range for a mechanic to buy & install a quality made new alternator on a 350 chevy engine, of '86 vintage ? Just want to have an idea of the cost to R&R before walking blindly into the shop to have this looked at soon. Thanks for any help from experienced mechanics out there.