I don't like this idea mainly because there are only like 3-5 champs who actually get 100% value out of every stat. Most champs build it because it's efficient enough for the 2/3 stats they use with a side bonus of incidental AP or attack speed they can't fully utilize. By making it more expensive and leaving its max gold value where it is, you're drastically narrowing the pool of champs who will actually build the thing. The way we're headed, it's going to be an item that exists only for Kayle and Jax.
The more I've thought about it, the better I like this idea:
+
+ 550 recipe
40 AD
60 AP
[Passive] Guinsoo's Rage: Basic attacks and abilities grant 4 AD, 6 AP, and 8% attack speed for 8 seconds, stacking up to 5 times. Upon reaching 5 stacks, basic attacks deal 10% bonus AD on hit as magic damage, abilities deal an additional 15% AP as physical damage, and gain Exhaustion after 3 attacks.
[Passive] Exhaustion: Guinsoo's Rage cannot be stacked and loses 1 stack per second. Lasts (15-20 sec).
Numbers negotiable, but I did math out these values. The idea here is to have a high-cost hybrid core item that can be used by casters or basic attackers to create windows of power. It becomes very gold efficient during short periods but is slightly inefficient outside those windows.
The good thing about this sort of design is that it has counterplay built in and it forces you to make choices about how to utilize your buff. Since it's not constant, the enemy can stall, bait, or catch you with Exhaustion up. It's great for pushing a turret without the Jax/Kayle problem of it always being ridiculously strong at pushing every turret. And it encourages choices between using buffed defensive/healing abilities or pressing your offensive advantage while it lasts.
It'll never happen, but I think this is the better direction that could work for hybrid champs across the spectrum of classes instead of the few who cheese the fuck out of all the stats all at once.