First point to go to is Zilog, there're other organizations who have license for the core. What I got from interacting with them is that they may not be interested in the deal unless it will bring significant dividends for them. You should take it into account because before you even start to discuss numbers and deal in overall you should be able to convince them that your implementation is worth talking and thinking about.
Next, as I see from the comments to the question, you are going to make DSP device. Some DSP capabilities (e.g. ADC/DAC) may not be properly implemented in FPGA, thus FPGA has relatively limited application here, and you will need another entities (on the board or in silicon) providing these functionalities.
However even with multiple chip design it will be cheaper and will provide speed required for video DSP. For that you can use any modern FPGA, and select ADC/DAC carefully, and, what's most important, make proper board design which accounts for high digital speed.
If you need 32K buffer for your video processing - FPGAs are having such memories which can work @200 Mhz, should be enough for more or less sophisticated digital signal processing (given, again, you select RAM organization properly).
Finally, you can make your own implementation of Z80, without reference to the original design, only utilizing command set and operational structure, and even not accounting for native Z80 timing making you implementation running at much higher speed than 20 Mhz which is a maximum for Z80. It is not a hard work, but after you make such implementation you should be able to write microcode in Z80 assembler, which may be harder given capabilities of the debugging utilities which you design.