If you use an Alveo 250, wich contain 1300000 LUT, as a SHA256 hash need about 1300LUT and 65 cycle, if your alveo is at 500MHz you will reach 6.7Ghash/s <1USD/day (alveo 250 is more than 1USD/hour on amazon).
An ASIC can do 110Thash/sec.
Even if you can optimize the sha256 algorithm, you will never can win money with fpga...
To those who thinks ASIC has a chance, they should know that in the end the speed is limited by the Source Synchronous communication (PCIE) between the main processor and any hardware accelerators (AISC,GPU, FPGA, etc). So speed of the ASIC won't benefit that much.
See the Jungle, not the trees