That's not really a cost-of-carry (in the financial sense), because it is independent of the value of the patent; It's just a delayed payment. And really, it is small - $10K over 20 years, some of it delayed (and you can "back out" in the middle). If you use professional help (a patent editor or patent attorney), you'll pay them at least as much, often 3 times as much, and you would do that upfront.

By cost-of-carry, I mean that if a patent is worth $1B, it should bother you like any other $1B asset. Are you aware of any other $1B that you pay (on average) $500/year to maintain, and yet get police forces all over the US (local, feds, custom officials) to underwrite and execute said ownership for you?