I am looking for some inexpensive 300 Win Mag Ammo. I will only need a light bullet for now since I will only be hunting whitetails this year.

Any suggestions?

Remington Core-Lokt usually runs a bit less expensive than, say, Winchester Ballistic SilverTips, and is available in 150 grain loadings. I'll let the .300 WinMag owners comment on it's performance, however, as I've not personally used a .300.
Good luck...

Reload your own, it's the only way I can shoot as much as I do. $6-7 a box

As NattyBumpo said. You won't realize the potential of a magnum without reloading; as there is little in the way of really accurate loads out there, without paying a premium price. But buying cheap ammo is a good way to gather brass to load and get some practice in.

The cheapest factory loads for the 300 win mag will most likely be at Wally World. Generally it will be Winchester X, Federal Classic, Remmington CoreLokt, or UMC. However, you will likely find them mosly loaded with 180gr bullet. Anyone of those rounds will work good at fouling your barrel.