I think if your going to compare the BBC costs you need to add the costs of PBS and NPR together
You can't even really effectively compare them that way.
NPR is slightly more comprehensive than PBS, since they
do directly produce some content, but it still just doesn't work the same way as the BBC system. The American public broadcasting system is based on independent local broadcasters. So, for example, one of the best programs on "NPR" that I like to listen to is Radiolab. However, Radio Lab isn't produced by "NPR" (which does produce some news content itself). Radio Lab is produced by WNYC, the New York City public radio station itself, and then is syndicated across the entire network (if the local station in question chooses to carry it). So, the costs to produce Radiolab wouldn't be included in NPR's numbers, they'd be included in WNYC's numbers. Another high-profile radio example is This American Life, which is produced by WBEZ out of Boston and is distributed by PRI, not NPR. NPR primarily produces their two big daily news programs:
All Things Considered (evening) and
Morning Edition (obvious). They also do special event coverage, like covering elections and things, and a few other shows like
Wait, Wait, Don't Tell Me and
Talk of the Nation/Science Friday. Everything else they "run" wouldn't be in their numbers (except for distribution costs, which are obviously not comprehensive).
Similarly, PBS doesn't "produce" the majority of the content that is distributed through the public television system. They are primarily a distributor of content produced by third-party sources such as affiliate stations, American Public Media (the largest one), NETA, and independent producers. Even what is probably the "biggest" show on PBS, Sesame Street, isn't produced by PBS. It is produced by Sesame Workshop (formerly Children's Television Workshop) and distributed in the US by PBS. In fact, unlike NPR, PBS does not even have a content-production "arm" at all. All of the content distributed by PBS comes from third-parties, with only a very few small (and occasional) exceptions.
To really evaluate the size of the US Public Broadcasting system, you'd have to evaluate the total expenditures not only of NPR and PBS, but of NPR, PBS, APT, APM, NETA, the Corporation for Public Broadcasting, the portion of PRI stuff that is American-funded, and every single public broadcasting station in the US (or at least those that do produce content, which is pretty much all of them, even if only for local audiences). And that doesn't even count all of the independent content producers. For example, many of the episodes of PBS's NOVA aren't produced by PBS itself (most, even). They are produced by independent filmmakers (often other, separate non-profit foundations provide the funding and hire out a production company), and then PBS chooses to carry a particular program. So, if a particular piece is funded by the Daughters of the American Revolution, and run on PBS, do you count them as part of the "network"? What if they do it regularly?
Since it is built on independent, locally-controlled affiliate stations and pools of available content (some of which are produced by BBC), rather than a monolithic institution, it is very difficult to easily quantify with readily available figures.