Which ink/toner does the Brother MFC-7860DW use?

The Brother MFC-7860DW (laser, 2010) uses TN-450, TN-420.

Compatible cartridges

CartridgeColorYieldPagesOEM priceCost/page
TN-450 black XL2600 $58.00 2.2¢ OEM · AmazonCompatible
TN-420 black standard1200 $45.00 3.8¢ OEM · AmazonCompatible

XL/high-yield cartridges usually cost more upfront but lower the cost-per-page — better if you print often.