These numbers are based off the current price of hashrate. As soon as you try to buy significant amounts for larger currencies like Bitcoin, the numbers skyrocket.
The nice-hashable column on the site shows how much hash power is available for purchase.
To read the page naively would be a little like claiming one could hoard 51% of the oil supply given today's price at the pump. In reality, as soon as you started buying in large quantities, the price would skyrocket (making that attack very much more costly), suppliers would cut you off, and others would notice.
it wouldn't be a flash crash if there was no demand. it would be a crash. the flash crash happened because there was insufficient liquidity on one exchange - as soon as the market sought to fill the gaps, it went right back up.
you could also "make change" by exchanging ownership of the $100 address for 5 $20 addresses, and change those down into 10 $2 addresses each. there's very little cost of executing this algorithm several times over for a "single" purchase.
There are problems possessing 10 $2 bitcoins, due to the minimal transaction fee (approximately $.20). Reclaiming the value (putting back On-Blockchain) of $20 worth of $2 dollar addresses will cost you 10%. Then again, %10 is how much Coinstar charges to turn loose change back into dollars. Still, a better alternative is this; replace Bitcoin with a lower-denomination Altcoin (Dogecoin?) to cover preset currency addresses below a certain amount. Dogecoin's transaction fee is negligible, so the reclaiming cost is no longer an issue. Though Dogecoin's fluctuations demand some level of, the lower-denomination restriction makes these fluctuations negligible (By which I mean this; A quarter is more likely to fall out of your pocket than a $10 dollar bill out of your wallet. However, the quarter's ease of use is more important than its potential loss of value, which is relatively negligible when compared to the $10 bill)
Even in it's current form, your grandma's wallet balance would resemble that of her BoA checking account, where the spendable amount would be a total of the easily-redeemable (Pay to pubkey hash) transactions.
More complex transactions would show up in more advanced wallets (possibly separately), and that is the way it happens in the real world now. Say you hold an options contract or a bond on some stock exchange - it's "spendable" but doesn't show up on your everyday bank account balance. This is useful money as we know it now. The allowance for such flexible transactions is a feature and not a weakness :)
In fact, banks here in Portugal already show you two balances, an "accounting" balance which shows you the sum of all transactions, including ones that haven gone through yet (e.g. deposited checks) and an "available" balance which is what you can actually use right now.
while in theory possible, this approach has not been done because it is said to be a slippery slope down the lines of being against "bitcoin neutrality".
once the feature exists to treat one set of coins different from another, it leaves the protocol open to control by centralized parties - they could ban the use of "unidentified coins", etc.
The nice-hashable column on the site shows how much hash power is available for purchase.
To read the page naively would be a little like claiming one could hoard 51% of the oil supply given today's price at the pump. In reality, as soon as you started buying in large quantities, the price would skyrocket (making that attack very much more costly), suppliers would cut you off, and others would notice.