The question seams to imply that there was a need for a ROM in the same sense as it was used in the 1970s and 80s, but why wasting address space and money on storage that could only be used for one purpose?
Machines, way passed the mentioned cut of date of 1970, used magnetic media (drums) and magnetic core for RAM. Both are persistent storage, this means information stored there will be present even after a power cycle. All a CPU needed to do is starting to operate from that memory. Data processing and scientific computing relied on having RAM for changing tasks, while process automation as well was happy with loading core once a while and after that simply used it ROM like - some machines had even ways to switch of writing of (new) data, essentially turning core into ROM.
Long story short: Prior to the 1970s there was no need for ROMs in the sense we know today.
Applications of ROM like storage was, if at all, needed in micro program storage, but solutions here were rather specific to each CPU manufacturer and machine, like CCROS, TROS and similar (*1,*2). Also, noone really wanted ROMS. In fact, this is the very reason why IBM developed the floppy drive: to load the microcode from a persistent media to micro code storage. That way they could use RAM,but kept the content static - and change it when needed.
All ROM like solutions of the time were special to type and hand made.
ROMs as we know now grew out of decoders. Fixed function devices translating information - like turning a key press into ASCII. A job that for early terminals was done literally with piles of diodes (*3). But even this use did only come up after 1970. As only then the need for high volume production, i.e. more than a few dozend, became a thing.
was cheapest per kbit before Year 1970?
Since there were no common, interchangeable solution before 1970, but only partitial solutions specific to each device, comparable prices can not be given.
*1 - I remember a TROSS like system using modified punch cards instead of special mylar sheets. While not as dense, they had the advantage that ROM content could created by using a standard key punch. Thus patching a ROM was simply DUPing the card until the change, enter the new value and DUPing again until the card was done. ROM changes within minutes, needing only equipment available in any computing center - beside the special cards that is. Even more, a new ROM stack could be delivered as tape and outputted by using a standard card punch.
Depending on one's POV, this may qualify as 'cheapest', still, it's a low volume hand made thing.
*2 - Core Rope Memory was as well a very specific niche solution
*3 - I remember that in 1977 I got hold of two scrapped terminal keyboards, which served as my main source for diodes for several years, as every key was connected to a row of 0..7 diodes to encode its ASCII value.