FPGA under $10 ?
Sure, plenty of choices under $10, but....
... I have found EP4CE15F23 (343 Ios and 484 pinsBGA) as an equivalent by looking into cyclone FPGA family...
... if any other vendor provides an FPGA with the same pin count and price under $10...
Welcome to the FPGA price curve, where more pins, and more logic (surprise) costs more.
If you really need those 343 io's
and the 15408 Cells, then do not expect a very low price.
eg Lattice have LCMXO3L parts with slightly more IO, but less Logic, and they have LFE5U series that start with quite good $/logic values, but the lowest cost (sub $10) ones do not have 343 io