The short answer is that accurate estimates of the numbers are hard to come by for just about any issue involving the US-Mexico border. This is in part due to the intense politicization it has received in the media and in the particular case of the 1930s repatriation, partially because it was not an official policy of the United States federal government. As the wiki mentions, estimates vary widely based on who you talk to. California estimated 2 million repatriations, which is on the very high side of estimates. The Tarnished Golden Door: Civil Rights Issues in Immigration, a report published by the U.S. Commission on Civil Rights, estimated a quarter of that number, but with a similar estimate of the citizenship rate.

In fact, 50-60% is the typical range for all sources, but this hides a rather subtle issue: Almost everyone is deriving their numbers from the same few sources. These are Decade of Betrayal and Unwanted Mexican Americans in the Great Depression. Neither of these sources provide adequate sources for their numbers and in turn rely on media reports and local governments for their numbers. We simply don't know how many people were repatriated in the various programs. The general range we do know is that 1,000,000 people is likely a far overestimate and 300,000 people is likely an underestimate. Hoffman argues for 400,000-500,000. Mercedes Carreras de Velasco^1 argues for slightly more than 300,000, but restricts her work to a smaller time period. Alanís Enciso found numbers similar to Hoffman, but accounts for pre-campaign repatriations by discounting earlier years, resulting in a total estimate of 425,000. If we go by immigration policies in previous decades, we're limited to a mere 100,000 repatriations, well below even what the Mexican records indicate.

There isn't really a definitive way to answer your question. Half a million people may be slightly on the high side, but we know that it was likely more than a quarter of a million. We similarly don't know how many citizens were included in these numbers. On the low side, there might have been as few as 125,000 or on the high side, perhaps as many as 300,000.

^1 Los Mexicanos que devolvió la crisis 1929-1932, Tlatelolco, 1974, pg. 65