I am wondering if there is a big difference in ammo accuracy? For example my gun in sighted in with Winchester supreme elite ammo. The question is is there any other ammo that will be dead on with where I have it sighted in? 150 grain.

Anything is possible but there's a slim to no chance; at least it had never happened for me. It would be in the same ball park generally but I wouldn't risk it.

I just really don't like lobbing $60 a box ammo down range.

There is no way to know unless you try out a bunch ammo. Sometimes even the same ammo shoots diffently between lots.

Do most ammo with the same grain have the same amount of powder?

Each ammo maker uses different powders and recipes. You have to determine what your gun "likes".

And you have to lob a bunch of dollar bills downrange to find out about that.

If you want to shoot something cheaper at the range just go ahead & leave it sighted in for your hunting ammo. Just shoot for groups at the range & don't worry about the exact point of impact. It will probably be pretty close, anyway at a hundred yards.

Don't know if rifle makes a difference but I'm shooting a sendero.

The short answer is, "probably not." See the thread "ballistics?" below.

If you need help with some 300 Win Mag ammo, let me know. $60 a box ($3 a round!) is crazy expensive for 300 WM. If there's one caliber I know well, it's this one. I shot it for years in competition.