Definitely make a list of everything you have in your pantry and freezer and then have a think about what meals you can make with them and if anything can become a meal with just the addition of one or two things.
Dry pasta is incredibly cheap and only need a few ingredients to make it a nice meal.
I'm going to go against the grain and say meal planning in advance only makes me waste things. I prefer to buy 'on the day'. I buy less and get a proper idea of how much each meal costs - which is handy to know for future lean weeks.
Of course it's not always practical to shop daily so do what works for you.
Oh and FWIW, I don't think $300 a week as shocking as most other people seem to!