Historical price charts for RAM are quite readily available, e.g. in the mid-seventies a ballpark figure was a penny a byte. What was the price of ROM (assuming you were getting the chips produced in quantities at least a few thousand) in that time period? I imagine it was cheaper than RAM, but by how much? (Every Google search hit I find, assumes I'm misusing ROM to mean anything from flash memory to magnetic tapes, which is a weirdly rapid timescale for a term to decay.)
You can get a sampling of prices for EPROMs the hard way, by checking ads from electronics magazine archives. For example on page 116 of Popular Electronics Jan. 1977 you can see the 1702A 2K(bit) EPROM listed for $12.95. The old magazines always had full-page ads with components near the end.