105k miles buys 24 500-mile upgrades. That's a value of $0.007/mile (compared to paying $30 each for the 500-mile upgrade); most here would consider that a terrible value. (I typically redeem at about $0.015-$0.02/mile for international award travel in coach; most here get far higher "value" by redeeming miles for international premium travel. I would never pay the fare for a premium cabin so I don't consider premium cabins a higher "value" for my money/miles, but that's a personal choice.) Either way, buying 500 mile upgrades with miles places a very low value on your miles. However, if you're not going to use your miles for anything else, I suppose that's better than not using your miles at all.

Note that you won't necessarily even be able to use your 500 mile upgrades, since upgrades are still subject to availability. You may well find better success as a Gold on AA with upgrades than as the equivalent on airlines with "unlimited" upgrades, but you probably won't have much success if you're flying on even moderately elite-heavy routes at moderately peak times. Again, upgrade success rates are an oft-discussed topic here (see a thread that's currently on the front page) and vary wildly.

Because of the uncertainty, buying 24 500-mile upgrades at once as a Gold seems like a big risk; you may or may not get to use them. Given that the cost per upgrade is 4375 miles or $30 (online, $35 from an agent) no matter how may you buy (no bulk discount), I don't see any reason to stock up at once.

Again, this is a personal decision. Roughly 2/3 of my miles are on flights that earn mileage towards upgrades but can't be upgraded (Eagle, Alaska Airlines, international), so I choose only to use the upgrades I earn (or buy a small number when I'm just short of upgrading myself and a companion). Others will buy the upgrades they need to upgrade all eligible flights.