Not worth it…!
The cheapest solution is the MC Cthulu by itself.
And yeah, you’ll also have to buy the RJ-45 passthrough jack ($8-$12 online) AND RJ-45 cables for various systems you want to use the joystick on unless you make those cables yourself.
Keeping an MC Cthulu in a joystick with an existing PS3 PCB – very redundant. Aside from Turbo functions and lack of “DLR” capacity, the MC Cthulu does everything the stock PS3 PCB does… and has the PS1/PS2 backwards compatibility in addition at least 12 other systems. And is a hell of a lot more convenient to use with PC emulation, too. And you don’t have the issue (SO FAR!) of a Cthulu-class PCB not being compatible with Mac, PC, or model of PS3 just 'cuz of some stupid graphics card/motherboard/CPU issues.
There was never an add-on PCB made to make PS3 controllers backwards compatible. This only happened with the TE Kitty and Kitty VLX PCB’s (for the 360) because A) the 360 market for dual-usage/PS3 compatibility was huge; and B) the Cthulu had already developed into a multi-compatible version that could be repackaged economically for the 360 add-on PCB. It was never going to be “worth it” for any PCB developers to make a PS3 add-on PCB for backwards compatibility.