From the book, "Farmers of Forty Centuries," which covers a 1909 trip to Asia by an American agricultural scientist:
Cattle were grazing among the graves and with them a flock of some 250 of the brown Chinese geese, two-thirds grown, was watched by boys, gleaning their entire living from the grave lands and adjacent water. A mature goose sells in Canton for $1.20, Mexican, or less than 52 cents, gold, but even then how can the laborer whose day's wage is but ten or fifteen cents afford one for his family?
I believe the reference to "$1.20, Mexican," refers to a Spanish dollar, which was the international currency of the world through the 17th-19th centuries.
But what do you make of "52 cents, gold?" Are these United States cents? Why gold?
China remained on the silver standard long after most countries had switched to the gold standard. A 'dollar mex', as it is usually referred to in Shanghailander documents, was the 8 reale or 1 peso piece minted in the former Spanish colonial possessions in Central/South America. Produced in various mints, they were shipped from South America to China via the Manila galleons to pay for tea, silk, and porcelain.
The 8 reale coins themselves weighed 27.07g, and contained 90.27% pure silver. The 1 peso coins, introduced in 1869, were identical. The coins thus were 24.43g of pure silver. The US dollar itself was defined in 1792 as 24.057g pure silver, based on worn 8 reale coins that Hamilton had in his possession. Incidentially, Spanish colonial silver reales and gold escudos circulated in the United States until the 1850's. So, an 8 reale coin would be worth $1.0157 USD, and passed at par.
The change came during the 1870's after the massive discoveries of silver in Carson City and other locations. The increase in the silver supply led to decrease in its value relative to gold, and the longstanding 15:1 gold:silver ratio changed to 16:1. In 1873 silver was demonetized in the United States and other major economies, leading to a defacto gold standard. The only major economy to remain on the silver standard was China.
Thus, China still used silver 8 reale coins (dollars mex.) while the rest of the world had switched to a gold standard. Economies like Mexico, the Philippines, Japan, and the Straights Settlements adapted a set exchange rate to the gold dollar, unrelated to their historical ratio. The net effect was a devaluation of silver coinage by roughly half.
So, by 1909 China was one of the only places still using the silver standard. To add to the confusion, most Chinese reckoned silver weights using the tael, roughly 40g silver. The Canton tael was 37.5g, or 1.5346 'dollars mex.' This also varied from treaty port to treaty port, with the Shanghai tael being worth 34g, or 1.38 Mexican dollars. The value of the 'dollar mex' against gold currencies varied daily, and Shanghai newspapers of the day would have a table of rates printed on the front page. Foreigners in the Shanghai concessions were just as confused, most being paid in x taels per year, but spending 'dollars mex.' daily. A 'chit' system evolved, where your club/restaurant/laundryman would bill you monthly, and your company accountant would deduct your spending from your salary.
So to sum up: $1.20 'mex.' would represent 29.32g silver, or 0.6516 Canton tael. At the time of publication, a 'dollar mex.' must have been worth $0.43, or 0.02g gold. ($20 = 30.08925g gold until 1933.)
Sources:
Elwell, Craig K. "Brief History of the Gold Standard in the United States." Congressional Research Service, 2011.
Bailey, Warren and Zhao, Bin. "Familiarity, Convenience, and Commodity Money: Spanish and Mexican Silver Dollars in Qing and Republican China." Cornell University, 2009.