The problem at 30 is that a team knows it's basically a last contract. You sign a striker at 30/31 for 3 years, and he is now 33/34, his form is likely declining, and no team will buy him for more than peanuts. Therefore whatever you pay for a player over 30 you won't recoup, so obviously teams don't want to spend big money on a 30 year old. The law of diminishing returns. Now a 27 year old you can still spend decent money on because hopefully at 30 you can sell him for about 1/2-2/3 of what you paid. Ideally the only striker you are going to spend more than 30 mil+ on is younger than 27. Like Edison Cavani at 26 for 63 mil.
So Mirko, who we bought for 15 mil at 28 is now 30 and worth at most 10 mil. 2/3. Quags who we bought at 27 for 10.5 million, is now 31 and realistically worth 5-7 million. 1/2 to 2/3. These guys have only a couple years of high quality football left and then are basically bosmans, or sold for peanuts. Why on earth would anyone spend big on a player who has no future resale value?