Unfortunately, there really isn't any lenses under $300 that will improve upon your existing lenses for nature and wildlife. Stick with your existing for awhile andave up the cash so you can get a telephoto lens with a f/4 or f/2.8 maximum aperture. The Canon EF 200mm f/2.8L II is an excellent lens if you're willing to go prime:
It's an incredible L series lens that's actually quite inexpensive for the quality of glass and an excellent performer even at wide open (f/2.8).
I had one and took great photos with it. I sold it after I acquired the 70-200 f/2.8L IS II.
Here's a couple of samples taken with that lens on a crop body:
The 70-200 f/4L is cheaper ($650 or so), but you sacrifice one-stop for the 70-200 focal range (compared to the 200 f/2.8L II). I actually had the 70-200 f/4L while I had the 200 f/2.8L II, but hardly ever used the 70-200 f/4L and would resort to the 200 prime and just zoomed with my feet.
After the 200 f/2.8L II, the next lens you'd probably want to go with is the 70-200 f/4L IS. This is an incredibly sharp lens and gets the added IS motor so you can handhold at slower shutter speeds (and hopefully the subject won't be moving).
As far as flash under $100, you'll have to look into the Chinese made Yongnuo flash units. I'm not very familiar with these, but most people who buy non-Canon flash units go with Yongnuo. Recycle times on these units tend to be noticeably slower than a true Canon Speedlite. But you get what you pay for. I have a Quanatray QDC-900WA that I'm looking to sell if you're at all interested. It's the correct voltage for Canon series dSLR and is E-TTL capable. Let me know if you're interested.