There are so many things that fit into this category, can you be more specific? Do you want to focus on supplies or durable gear? Do you mean the category that falls in between durable and expendable (like water bottles and such necessary gear)?

When you say "must have" implies to me that you are basing this on the survival priorities of shelter, water, signal, fire.

I would begin with a poncho. There is a wide range of quality and price, but a lightly used military surplus one will usually run about $20 (US) and last a very long time. It can be used as waterproof/windproof outerwear or as a shelter.

Next, you need the ability to store and carry water. There are many good water bottles out there, but I prefer the single walled stainless steel. These may tip the scales on the more costly end, however. Single wall stainless can be used in a fire to purify, will prevent solar radiation from making your water too hot to drink, and is durable.

A nesting cup, that your water bottle fits into, can make procuring and purifying water much easier, and cooking as well. They weigh little, take up almost no space and are very durable. The stainless steel ones are usually less than $20, though titanium may run over $30.

All of the best fire starting methods will be under $30: lighter, UCO stormproof matches, cotton balls, petroleum jelly, most ferrocerrium rods, and my favorite striker for the ferrocerrium rod, a cut down hacksaw blade. Even the Blastmatch, and its little brother the Sparky, are less than $30.

Almost every whistle and signal mirror cost under $30. There are some emergency strobe lights than are under $30, but the best will run over that price. Any brightly colored material (though we have discussed the advantages of orange and blue) with a large surface area can serve as a signal panel. The ubiquitous blue tarp will work well and make your shelter, for under $30.

The orange survival bag: Orange survival bag

or: Smaller orange survival bag

is inexpensive and works for signaling as well. Any of the emergency or space blankets will cost less than $30.