Selling, general, and administrative. Selling, general, and administrative (“SG&A”) expenses were $12.9 million and $11.5 million for the three months ended September 30, 2024 and September 30, 2023, respectively. The $1.5 million increase in 2024 was primarily due to the increase in stock-based compensation expense.
Other income (expense), net. Other income (expense), net was ($0.1) million for the three months ended September 30, 2024, compared to $11.3 million for the three months ended September 30, 2023. Other income, net in the third quarter of 2023 was due primarily to recognition of the $7.8 million legal verdict for contract (compensatory) damages related to the 2018 Elk Creek silo failure as well as the receipt of $3.0 million of insurance proceeds related to the methane ignition that occurred at our Berwind complex in 2022.
Interest expense, net. Interest expense, net was $1.7 million for the three months ended September 30, 2024 compared to $2.4 million for the same period in 2023. The decrease in 2024 was largely due to the repayment in full of related-party debt in the fourth quarter of 2023 associated with the financing of the acquisition of Ramaco Coal from Yorktown Partners. Interest expense related to this financing was $0.4 million in the third quarter of 2023.
Income tax expense (benefit). The effective tax rate for the three months ended September 30, 2024 and September 30, 2023 was 9% and 22%, respectively, excluding the impact of discrete items. The primary differences from the federal statutory rate of 21% are related to state taxes, non-deductible expenses, the foreign-derived intangible income deduction, and depletion expense for income tax purposes.
Earnings (loss) per share. Refer to Note 10 of Part I, Item 1 for information regarding earnings per share calculations for Class A and Class B common stock.
Nine Months Ended September 30, 2024 Compared to Nine Months Ended September 30, 2023
Revenue. Coal sales revenue for the nine months ended September 30, 2024 was $495.4 million, approximately 1% higher than the same period in 2023 due to the increase in tons sold, which was mostly offset by the negative impact of pricing. The 16% increase in tons sold occurred in both North America and export markets, with export volumes increasing by 20% and North America volumes increasing by 10%, and was aided by the Company’s increased capacity for production achieved during late 2023. Revenue per ton sold decreased 13% from $199 per ton for the nine months ended September 30, 2023 to $173 per ton for the nine months ended September 30, 2024 and was driven by the variability in index-based pricing for export sales. Revenue per ton sold (FOB mine), a non-GAAP measure which excludes transportation revenues and demurrage, decreased 14% from $169 per ton for the nine months ended September 30, 2023 to $145 per ton for the nine months ended September 30, 2024. Refer to Non-GAAP Financial Measures later in Item 2 for more information regarding this measure. U.S. metallurgical coal price indices have fallen by roughly 32% on a year-to-date basis driven by the macroeconomic conditions discussed previously. We expect metallurgical coal prices to remain volatile in the near term.
There are no revenues from rare earth and critical minerals at this time.
Cost of sales. Our cost of coal sales totaled $397.2 million for the nine months ended September 30, 2024 compared to $354.4 million for the same period in 2023. The 12% increase was driven by the increase in tons sold, as discussed directly above. Cost of sales per ton sold decreased 3% from $144 per ton for the nine months ended September 30, 2023 to $139 per ton for the nine months ended September 30, 2024. Cash cost per ton sold (FOB mine), a non-GAAP measure which excludes transportation costs, alternative mineral development costs, and idle mine costs, decreased 2% from $111 per ton for the nine months ended September 30, 2023 to $109 per ton for the nine months ended September 30, 2024. Refer to Non-GAAP Financial Measures later in Item 2 for more information regarding this measure. Mine costs for 2024 were impacted negatively by challenging geology and labor constraints in the first quarter of 2024 but improved during the second and third quarters of 2024 due to efficiencies gained from increased production compared to the same periods in 2023.
Depreciation, depletion, and amortization. Depreciation, depletion, and amortization expense totaled $48.9 million and $39.9 million for the nine months ended September 30, 2024 and September 30, 2023, respectively. The increase