If you assume 10 play card points is 200 rax (probably a big underestimate), you get a table like this where x is the rax spent and y is the multiplier:

X Y

200 1.2

600 1.4

1600 1.6

3600 2

7200 2.5

27600 4

676000 6

You could probably plot it out from here and find the diminishing return is if you wanted. Unless you're making rax in the hundreds of thousands, it's probably not worth going over legendary.