I bought the extended warranty when I purchased my gm vehicle new. I paid $1700 for a 6 year 90K zero-deductible warranty. Actually, I paid $1400, but they knocked the $300 I negotiated off the warranty price from the sale price of the vehicle because they said $1700 was rock bottom on the warranty. Yeah, right.
I was looking for a warranty for a used car we purchased recently and the warranty company has been sending me emails offering $300-$400 off the original price they quoted me. Maybe the GMPP company will do the same thing to you if you wait a week or so.
I too think what you were quoted seems high. My brother-in-law called around looking for a warranty on his GM truck and he got quotes from dealerships in the area of $1200-$1500. I don't know what the coverage was but I would assume he was going for the max. Good luck!