Reddit mentions of Emergency Mylar Survival Sleeping Bags - Pack of 4 Bags. SSB-4

A lot of this would depend on the situation, but assuming that this bucket would be all that I'd have to live on for 72 hours...

• Line the bottom with 1 liter bottles of water. I'd want minimum 6, preferably 9. Ideally I'd also like a Sawyer Mini Water Filtration System. That's a chunk of the budget right there, but it allows for refilling of water bottles from a lot of sources which could be valuable depending on the situation.
  
  
• 3 Mainstay 2400 Bars. Walmart sells them for less than $5.00 each. I'd add in a couple of gallon sized ziploc baggies to boot because they aren't individually sealed. Probably wouldn't be super pleasant to eat only these for 3 days, but there's zero prep involved and they get the job done. Maybe get an extra one to try out ahead of time to make sure they don't do anything funny to your stomach.
  
  
• Probably some mylar sleeping bags and several blankets. Then add in 100' of paracord. The blankets and paracord would allow you to provide shade or insulation as needed.
  
  
• Two lighters (one for backup) and a standard pill bottle filled with cotton balls smothered in vaseline (tinder for fire, cheap and efficient).
  
  
• Hand sanitizer (also works as a fire starter), sun glasses, chapstick, and a neck cooler (also sold for <$5.00 at walmart). Add in a handkerchief that you can soak and cover your head with. You can probably find more clothes to put on if it's cold, but it's hard to deal with heat, so that's important to take into account. Add in a small roll of duct tape as well.
  
  
• A mini-med kit. To keep it within budget I'd mostly use what I have at home. Add bandaids, gauze, lots of ibuprofen, lots of multivitamins, sinus meds, pepto tablets, sunscreen, and allergy meds. Splurge and pick up a decent sized tube of antibiotic cream. It's not comprehensive, but this is a 72 hour kit. Hopefully that would mean more being concerned with comfort than full on survival.
  
  
• A locking folding knife of some sort. Make sure it's sharp as a razor before putting it into the bucket. Doesn't have to be top notch or expensive here, but does need to be sharp. If room permits and you have one already, also toss something Ka-Bar like in. Something that has a little weight to take a branch off, but also is sturdy enough to assist in minor digging.
  
  
• If you have them, toss a pair of wool socks in. No matter the weather those are always good to have. Otherwise, just get an extra pair in there. If the weather is cold you can double up. If it's hot you can change them out to keep your feet fresh. That's important.
  
  
• A spare phone charger. Chances are there will be electricity somewhere, and phones are absolutely important. Worst case, get a $10 cheapy, test it, then toss it in.
  
  That's the necessary stuff for a 72 hour kit imo. It covers shelter, food, water, warmth (and cooling), and as much security as you can get within budget. It should keep you comfortable for a few days if you have to grab it and move toward a place where you cannot be assured of any necessary resources being present. You can fill gaps with things like gum, hard candy, a good book you don't mind re-reading, as well as anything else you might need to keep yourself occupied.