Quote from drownpruf:
Binaries are discrete vanilla verticals, so trade listed products. Trade OTC what can't be replicated easily, like DNTs. A 47x53 market is actually acceptable. Consider the market in a similar call or put spread. You're going to pay three or four cents on an SPY vertical.
Thanks drownpruf! I am considering this; or rather, after newwurldmn's + Martinghoul's insights into the institutional space, I will have to consider this!

SPY options' liquidity is indeed a boon.