I don't have extended yet since I just got my CPO car with 3 years left. But I researched some before my purchase. Ziegler is very good and web site is flexible, but for absolute lowest prices, I found grangerfordextendedwarranty.com is best. Their web site is not as flexible but has improved. For example, last I checked it does not have an adjusted price if car is a current CPO. But they will quote you. I got a quote by contacting them directly on 9 year /100k warranty (CPO + 3) from them directly at around $1000, and 9 year/125k at around $1450. (total years/miles from in service). The CPO makes the added warranty cheaper by around $1000 - $1200 I found. Ziegler had a "payinfull" discount code of $275 and is a good option too.
Be aware that for CPO cars still under warranty, you can extend to 11 years total (wow) at a reasonable price. However, above 100k miles total prices go up quite fast. Toward end of CPO I will consider where I am.
My
Cadillac is 8.5 years old/100k miles, had just $200 in repairs. On the other hand. my Lexus LS430 had cpo + 3 (10 from new) and great as the car remains, it had plenty of covered issues: mirror, cruise computer, air strut, etc. 0 deductible was nice.
1 more thing: every time you go to a dealer they will likely warn you of some astronomical diagnostic fee ($225 recently, I guess 1 hour of labor) that you must agree before they even start the diagnosis. It is waived if repair is confirmed/warrantable. So it feels like a crapshoot, but most genuine issues are covered. My brand new to me CPO had a purge valve go bad after 2 weeks, so 0 cost (emissions warranty, not CPO) but it's a slight gamble until they diagnose it.