I don't know about mcmillan stocks but the lightweight fiberglass stocks I have put on my rifles don't cost $800, I guess if you got the top of the line stock custom installed on your barreled action the bill might soar that high, however I do all my own stock work so Its just my time and some materials and the cost of a stock blank which varies from company to company. Normally a decent stock blank will run about $300 some of the ultralite ones will cost a fair bit more but also weight a fair bit less.
Right now I'm sourcing out a Kevlar/graphite or carbon fiber stock for my Custon Titanium to reduce the weight even further and replace the Hi-Tech sheep hunter that I installed on it earlier this year. And as we speak I'm installing a different HI-Tech specialties stock on my 375 H&H M700.
As others have said, a properly bedded rifle will shoot the same regardless of what it is made of provided its a stable material. The added bonus of a good synthetic is Lighter weight, reduced recoil(in some cases), a stable weather resistant platform that won't warp, and its a nice custom feature for those who take pride in there firearms or need there firearms to serve a specific purpose.
The factory Ti stocks are a nice stock but I find that they feel a little funny and "straight" and I also found that they transfer recoil VERY effectivly right to your shoulder joint, my factory 270 TI recoiled WAY harder than my 30-06 mtn rifle with a Hi-Tech stock and the 30-06 was shooting a 168 gr bullet vs a 130 for the 270.