$100 to spend… on what?

$20 shipped for Sawyer Squeeze Mini right now, see the Gear Deals section.
Minimalist trail running shoes will be around $50-$60 on deal websites like Sierra Trading Post, The Clymb, etc.

Then you'll still have some money left over to throw towards pants or a jacket.

Sawyer Mini – $20 shipped @ Lawson Equipment. Although I got mine from somewhere else, I'll plug for Lawson. Can't beat it for the weight/performance/$.

trail runners. The weight gain could be huge.
Always try. Shoes are very personal.

If I had to pick just one: Trail Runners. I like the Salomon XA Pro 3D 2 but everyone's feet are different so you have to see what works for you. I think they retail for $130, but I can find them for less, sometimes even as low as $60 for last years colors. If you can do that you still have cash left for:

A Sawyer Mini($20) or Aqua Mira Drops($15) to replace your filter.

Not sure what kind of Jacket you are talking about, but if its a rain jacket, you might try Dri-Ducks ($20).

Pretty happy with my OR Helium 2. Can ususlly be had for 100$ and comes with a great warranty.

Where do you usually backpack and whats the weather usually like ?

$20 Driducks, if it suits you, are a great lightweight options like others have stated.
$20 sawyer mini is great too.
$45 Merrell mix master are pretty good trail running shoes.

less than 100 dollars you replace them all.

$50 for a full tank of gas and the other $50 for beer and pizza on the way back.

Go to your local shoe store and pick out the cheapest pair of lightweight running shoes that are comfortable for you. You might even be able to get away with $35 for a pair.

Better yet, if you already have comfortable running shoes then use those first.

Forget all the marketing about shoes, it's mostly BS. You don't need "trail runners" if you don't want them. They aren't that much better than road shoes anyway, and the mark-up can be substantial.

The 4 most important things about any trail shoes are:
1) They are comfortable (most important thing!)
2) They are lightweight (shoot for under 2 lbs for the pair)
3) They dry quickly (do not get "waterproof" shoes, they are hot bathtubs, it's a nightmare!!!)
4) They don't disintegrate after 100 miles (look for shoes that have a good toe bumper and durable material where the upper meets the midsole, as this is the biggest point of failure and is likely where your shoes will be poked the most)

Next, buy some Aquamira drops for under $15 and leave the heavy filter at home.

Next, buy a GoLite chrome dome umbrella for the rain for $25

Put the last $25 in your savings account if you haven't spent it on shoes.

Just my two cents.

Alex Wallace for the Win!!

+1 on a trip
Skip the beer and pizza, get two tanks of gas, and go somewhere a little further that you've never been.

Nm

for water treatment is in your budget. Decant into mini bottles for an even lighter option for shorter trips.