Method 1 (I highly don’t recommend this for safety reasons): Monkey bars

 

You can go down to the playground near your home and hang upside down by hooking your legs over and under the monkey bars. Just make sure that your head is at least a few feet off the ground when you’re inverted. For weights, well, since you don’t want to spend a dime on anything, then I guess you’ll have to bring your textbooks down to the playground with you lol As embarrassing as that may be, it’s not so bad. I mean, look on the bright side. You don’t have to spend any money at all :D

Oh, and it’s advisable for you to have a spotter present ie. someone to watch over you in case things go horribly wrong. Your spotter can also pass you the weights or textbooks when you're inverted. Of course, that spotter should preferably be someone that you trust. If he’s the type that looks away every 2 secs to check out the girls walking past with their thongs sticking out of their jeans, then he probably wouldn’t make a good spotter.

Right below is a vid of someone hanging upside down from monkey bars. Notice the way  she hooks her feet over and under the bars. Oh and the head banging is quite funny lol
  Pros:

  •          Absolutely free

Cons:

  •          Not really safe. If you jerk too much while doing these inverted sit-ups, your feet might slip and of course, you’ll end up looking like a twat to those toddlers playing on the slides. Oh, the fractured skull is a bonus ;) Same thing will happen if you get tired and can’t hold on to the bar with your legs.
  •  Might be a tad inconvenient, having to go down to the park, especially since you’ll need to do this at least twice a day.
  •         Some toddlers might be playing on those monkey bars so you’ll have to wait for your turn lol Yeah, no jumping of queues or those little buggers are going to gang up on you and beat you up while screaming “Go go, Power Rangers!” at the top of their lungs.
  •        You’ll get weird stares from the people there (but then again, if you’re really concerned ‘bout saving money, then a few stares from strangers shouldn’t bother you too much.)
  •       Uncomfortable to hang for longer durations.

Again, let me reiterate. I highly don’t recommend this method for safety reasons. However, if you’re really desperate to save money and your legs can grip rather well, then go for it. Just don’t blame me if you fall and wake up in a hospital, not being able to remember that you’re once a highly-trained assassin for Treadstone.