Those numbers don't make sense. If you follow the chain of references, they seem to be made up.
(Why do I say the numbers don't make sense? a) That subsidy works out to about 35 cents per person per day. How could that possibly reduce the price of a pound of hamburger by a factor of 6? b) In other countries, a Big Mac costs at most $6.71. The $13 figure seems to be nonsense.)
I agree the reference is weak and there is most likely a large amount of conjecture behind that number, but the numbers don't a priori not make sense.
Subsidies can have both > 1 multiplier impacts and non-linear impacts. For example, subsidies allow for artificial economies of scale to kick in that can then bring the price down significantly. However, were those subsidies to dry up, then certain thresholds may no longer be met and you may see again a non-unitary and non-linear rise in price.
Indeed this is the main theoretical reason why a society would be interested in subsidies: they can get back more benefit than what they pay for in the form of the subsidy.
AFAIK there is no subsidizing of the beef industry in the US directly. There is subsidies for the sales of common feed plant crops and that's an indirect subsidy for growing beef, but that also applies burgers made directly from plants.
This is dwarfed by feed subsidies. Feed subsidies don't convert directly to edible-food subsidies; the 'big five' staple crops are subsidized independent of 'specialty' crops, and the vast majority of corn is grown for feed and ethanol in the US, and is of a variety not used for any kind of human food production.
The corn for feed is just used as a protein source though. I'm not sure why you couldn't grind it up for use as a protein source in say a plant-based burger.
(Why do I say the numbers don't make sense? a) That subsidy works out to about 35 cents per person per day. How could that possibly reduce the price of a pound of hamburger by a factor of 6? b) In other countries, a Big Mac costs at most $6.71. The $13 figure seems to be nonsense.)