I did mine on the cheap buying a used partial kit off ebay and doing it myself. My bike came with the 6800 series Ultegra so the 6870 kit I bought matched everything I already had. I had to get creative with the installation as my bottom bracket shell is a piece of aluminum tube molded into the frame so I couldn’t run the junction b out the bottom bracket to hook it up. Ended up running everything up the seat tube and stuffing it back down. Didn’t rattle the few times I’ve been out so I’ll call it a win.

I will say it’s not for the faint of heart as it requires a bunch of putzing around to get everything where you want it but in the end I’m so glad I did it. Assuming you Madone is internally routed you’ll likely have an easier time as I believe they use the same from for mechanical and di2, whereas mine was only offered in mechanical so the provisions for di2 weren’t there.