reasons? for example
6 layers at JLCPCB i cost about $42 and get about 7 pics.
The XC7K70T-FBG484 cost me $22, the biggest part of course.
The most expensive(for me) part for me is MAX9850 from ADI(Analog Inc. 's(include Maxim) parts are always of high performance and very expensive), about $3.
the FT2232H from FTDI.
and the 256Mbits HyperRAM.
Si5351 replaced to MS5351, at same performance and lower price.(In fact I choose 5P49V6965 before, but too expensive and doesn't provide too much improvement)
the other things are quite cheap. 1 dollar or less. 1 dollar, 1 dollar, 1 dollar...
for example MP2122 1pcs at $0.14, TPS62480 1pcs at about $0.7. SOIC-8 150mil slot(just the FRAM/EEPROM in the diagram, you can replace it from the slot)(from LOTES) is a bit expensive, but bellow or about $1.
of course you can unpopulate some of them.
In fact the biggest tough thing for me is how to solder the big BGA FPGA to the board......