Here's a very good scope for the money for $250:
http://www.brunoshooters.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=BSS&Pro duct_Code=WEO800473&Category_Code=35WVGS

Weaver makes an excellent scope, 3.5-10 would be plenty for a 300 mag.
If you insist on higher magnification you will have to pay more, its just the way the cookie crumbles buddy. Checks and balances... You want a good, reliable scope it will cost. You want a good, reliable scope, with higher magnification it will cost more.