There's lots of improvements you can make for 100 or less, it depends on what is the most important to you. If you're looking for power you can get a clamp on air filter and lose the airbox lid and rejet the carb. That'll be around the $100 mark if you do it yourself, and you'll get some power out of it and a safer, more reliable filter setup in the process. Another one of the most worthwhile changes I did on my warrior was a direct shifter, just makes it nicer to ride and gets all the slop out of the shifting process. If you hate the stiff clutch like most of us do, you could swap the warrior parts out for raptor 350 clutch release parts, it's a quick easy job, doesn't cost much, and makes a huge difference in the ease and smoothness of the clutch. Skid plates are another good place to start, the first thing I do when I buy a quad is get full body armor for it. At least the swingarm skid is a good investment, and will fall close to your budget. The stock one is only good for one hit before it's grinding against your chain and you need to beat it back in shape with a rock.