1) Club stores maximize your ability to buy in bulk. Repackaging bulk items in 1-2 serving sizes allows you to avoid throwing away uneaten leftovers, or they can be prepared and frozen per the recommendations above.
2) Frozen vegetables (no sauce) are a less-expensive alternative to fresh and can be prepared quickly. They lose some nutrients in the freezing process, but are nealry as healthy as fresh and require little preparation.
3) Tilapia is a fantastic fish for p90x because it is inexpensive, lean, usually sold frozen and packaged in individual portions of 3-5 oz, and it also has a very mild flavor that can be seasoned in a wide variety of ways.
4) Spend money on (low salt) seasonings, not on premium proteins. Season chicken breasts with chili powder and cumin, curry, italian herbs, lemon-pepper, or thai spices, etc, to add variety to “plain old” chicken. Scrambled egg whites can be seasoned in a variety of ways, too.
5) Water-packed tuna, especially low-salt tuna, is inexpensive, portable, and filled with protein and healthy fish oil. Similarly, egg whites are one of the cheapest protein sources available, if you can find ways to keep them interesting.
6) Shop sales. Clipping coupons will not help much on p90x, since most coupons are for packaged foods that aren’t on the nutrition plan. However, scour your grocery store’s sales fliers for sales of meat and veggies, and shop on the day the sale begins for the best selection (most store’s sale cycle begins Wednesday or Sunday)