Per this wiki article, though may not be up-to-date, the pay-scale for the Afghanistan soldier looks like this:
Under the US–Afghanistan Strategic Partnership Agreement (July 4, 2012), the United States designated Afghanistan as a major non-NATO ally and agreed to fund the ANA until at least 2024. This included soldiers' salaries, providing training and weapons, and all other military costs. (The majority of training of the ANA is undertaken in the Afghan National Security University. In 2019, the ANA had approximately 180,000 soldiers out of an authorized strength of 195,000.)
Soldiers in the Army initially received $30 a month during training and $50 a month upon graduation, though the basic pay for trained soldiers later rose to $165. This starting salary increased to $230 a month in an area with moderate security issues and to $240 in those provinces where there was heavy fighting. About 95% of the men and women who served in the military were paid by electronic funds transfer. Special biometrics were used during the registration of each soldier
Say the $300 million spending figure is accurate, so the average salary per soldier per month could be:
For the prevalent claim that there are 300,000 soldiers $/month = 300,000,000/(300,000*12) = $83 per soldier/month.
For the US planned 195,000 soldiers, $/month = 300,000,000/(195,000*12) = $128 per soldier/month.
As both salaries are within the salary range indicated in the article, so if there is an accurate headcount, say 100,000 active soldiers, the number of ghost soldiers then is likely between 200,000 and 95,000. However, the actual headcount is everybody's guess.