Man.... $75 is dirt cheap. I know I wouldn't touch it for that little, even if they're simple to replace. The risk of having to warranty the job, then the time it would take me to test the charging system before and after. A good tech should also road test to make sure you're not running into any problems. I say minimum time to change the 20 minute alternator is 1 hour once you've followed protocol. A larger shop or the dealer also has to pay the service writer for the time he/she spends with the customer and the parts guy for him to track down the part.
There's a reason the dealer is more expensive. If I were to take a shot at where their $800 estimate comes from I'd say it's $500+ for the part and $250-ish for labor, then tax (depending on where you are). Dealers are proud of their parts. Usually for good reason. The factory stuff is usually better than an off brand re-build where they change only the faulty parts.
I'm not saying it's right or wrong to have the jag mechanic fix it cheap, I'm just explaining a bit how a bigger shop works. Sometimes you can get a deal from one man gig, but usually they don't make it for long, or the owner/mechanic spends their whole life chasing their own tail trying to keep afloat.
Just be glad it's not a water cooled BMW alternator. The part alone is $800 and it's about 4 hours of labor to replace.
Cliff.... I don't blame you for not doing it yourself. There are times when I wouldn't do it myself. Life comes first. If I've got the time and nothing else going on, then I'm all for jumping on projects myself. Otherwise.......