Where does the phrase, "Costs an arm and a leg" come from?

The George Washington story and that of painters of his time who charged prices according to the number of limbs they were supposed to paint appears to be inconsistent. A more credible etymology is the following:

The expression “to cost an arm and a leg” is a metaphor about precious body parts. The similar line “I’d give my right arm…” dates from the early 1600s.

The phrase “an arm and a leg” rattled off the tongue easily before it was used to signify an exorbitant price. After the American Civil War, Congress enacted a special pension for soldiers who had lost both an arm and a leg. The phrase “cost an arm and a leg” begins to crop up in newspaper archives in 1901, referring to accidents and war injuries. In 1949, it shows up in the figurative sense.

The Long Beach Independent reported, "Food editor Beulah Karney has … ideas for the homemaker who wants to say 'Merry Christmas' and not have it cost an arm and a leg."

(mentalfloss.com)

Christine Ammer, American Heritage Dictionary of Idioms, second edition 2013) has this relevant entry:

arm and a leg An exorbitant amount of money, as in These resort hotels charge an arm and a leg for a decent meal, or Fixing the car is going to cost an arm and a leg. According to Eric Partridge, this hyperbolic idiom, which is always used in conjunction with verbs such as "cost," "charge," or "pay," an became widely known from the 1930s on, probably came from the 19th-century American criminal slang phrase, if it takes a leg (that is, even at the cost of a leg), to express desperate determination.

An Elephind search for "cost an arm and a leg" turns up this match from "Rex Beach, Novelist, with Sight Restored by Bride's Care, Will Take New Wife on New Hunt for Coveted 'Blue' Bear" in the New York Evening World (September 12, 1908):

Once there [in the Arctic], would you dauntlessly beard fourteen brown and black bears and grizzlies—all you find—and slay every bruin's son of 'em and so strain your tortured eyes in search of that blue bear's den that only five weeks of ceaseless care in a hospital would restore an almost helpless case of iritis—the most painful malady known to medicine?

Then, nursed back to life by that same little wife who had unconsciously set you revolving in a whirlwind of adventure and peril, would you still insist that were there but one of those blue bears in existence you would yet bring it home for that little blue room, though it cost an arm and a leg besides the eye?

The usage of "cost an arm and a leg" her appears to be literal (but exaggerated), rather than a figurative way of saying "cost a lot of money." Nevertheless, it suggests that "cost an arm and a leg" was already idiomatic in the U.S. in 1908.

The oldest match I could find for "cost an arm and a leg" in a purely figurative sense is from "Record Accessories," in The Record Changer, volume 7 (February 1948) [combined snippets]:

The finest needle on the market for some time now has been the diamond point. However, it had one serious drawback. It cost an arm and a leg, running about $50. The Walco people, realizing that the diamond, due to its extreme hardness and highly polished surface, did the least harm to record surfaces, strove for many years to devise a cheaper manner in which to polish the precious stones. They have finally come out with a diamond point needle which sells for about one quarter of the previous one. Now it only costs $12.50. This is still a lot of hay but one cannot expect the price ever to come down much below this. It's pretty easy to chip a sapphire but one would have to really go some to do any damage at all to one of these babies. Expensive but good.

Another instance, in the May 1948 issue of the same publication, reads as follows:

Several readers have written requesting that we mail them their copies either first class or airmail. I really don't know how to answer these requests. First of all it costs an arm and a leg to mail a publication these classes considering that the weight varies between 5 and 8 ounces. You fellows would have to pay it , of course. Is it worth it to you to do that? My own feeling on the subject is that unless all copies were delivered in that way it would give an unfair advantage to the fellows who spend the extra postage money.

I confirmed the dates of both instances at the Internet Archive.

Also, from an unidentified advertisement—possibly for Hoffman Motorcycles—in American Bicyclist and Motorcyclist, volume 71, page 63 (1950):

Distributors and dealers are invited to write us for particulars. Thousands of customers have been hoping for a quality motor job that would not cost "an arm and a leg."

Hoffman manufactured motorcycles from 1948 to 1954. A Hathi Trust search confirms that volume 71 of American Bicyclist and Motorcyclist was published in 1950.

In both instances from 1948 and in the instance from 1950, the "arm and a leg" are purely metaphorical.

Conclusions

It follows that "cost an arm and a leg" has been in use in the United States in an exaggerated but literal sense since at least 1908, and in a figurative sense (meaning "expensive") since at least 1948.