Boots– If you have any money to spend choose to use it for boots, they are the most important item, at least one decent pair of hiking socks will make all the difference too. Try a sports clearance centre, they usually have a good selection of inexpensive boots. Keep an eye out in your local thrift stores, you may just get lucky and find a pair in your size.
Water Purifier – This filter from amazon is cheap, robust and small to pack in to a bag. You can also get purification tablets, which are best value if you know you will mainly be using purified water and will only need them for emergencies.
First aid kit– Or at the very least a roll of bandage, antiseptic wipes and some painkillers.
Rain jacket– I found my North Face jacket in a thrift store in California, it was $10 and I always wear it hiking.
Pants & other clothing– You probably already have something you can use lying around in your wardrobe. Avoid denim as it is uncomfortable and tough to dry out, likewise cotton as when wet it makes the body cold and can lead to hypothermia.
Water bottle– A few dollars from a thrift store.
Day pack– Again, thrift stores or search out something you already have.
Means of starting a fire– (in a waterproof bag),
Spare batteries -for your torch, a map, knife and sunscreen.
Warm clothing – in a garbage bag to prevent it from getting wet in a survival situation.
Clif bars or energy gels will be necessary when stranded, lost or if a hike takes longer than you anticipated.
Emergency Shelter and a Space Blanket– seem annoying to add the extra weight, but it could be the difference between life and death