If the value of bitcoin is so low that you can't buy enough of them to pay for your purchase without drastically changing the value of the currency then they're not particularly useful as a method of payment.
At the moment of this writing, there are 13725450 Bitcoins in existence [1], with a value of $195.30 each [2]. That makes a grand total of $2,680,580,385 (2.6 billion).
How low would they have to fall for there not being enough of them to perform a transaction?
2.7 billion market cap, not available to you. Say you needed to make a $10,000+ purchase. If you can't buy or sell that much bitcoin without considerably altering it's price it becomes a hassle of a currency.
How low would they have to fall for there not being enough of them to perform a transaction?
[1] https://blockexplorer.com/q/totalbc [2] https://www.coinbase.com/charts