A 25GbE capable Mellanox card that also runs perfectly at 10GbE, which is how most people use it. Hardware offload support is stronger than the Intel X520, and the ConnectX-4 generation is new enough to stay supported in current kernels. Prices have fallen enough to make it a genuine alternative.
Across 100 listings tracked in the last three days, the median total cost in USD including shipping is $59, and the lowest is $25. RackSniper flags a listing once it falls at or below 70 percent of the trailing 30 day median, which for this product means roughly $41 or less. Condition and configuration vary widely on used enterprise hardware, so treat the median as a reference point rather than a target.