There's 3 headphones I almost always suggest for beginners. 30 dollars just wont cut it for mixing on any pair of headphones that I know of....including the 205s there mr.chemich. There's just too many flaws cheap material can introduce.
First has already been mentioned--the Sennheiser HD280's. They come off as a bit strong in the mid area, and are a bit tight if you have a bigger head. Both of these make them a bit more subject to ear fatigue then the other two I'm soon to mention. Despite that though, these are probably the most accurate playback devices of the sub hundred range, making them my top choice for the mixes sake.
Second would be the Shure SRH440s. These things are relatively comfy and can take a beating. They are Shure so they are American made and MILSPEC meaning they literally test this stuff in chemical warfare, shot with weapons, dropped from high distances, etc...on a military base. Very high security, very cool stuff. They are a little bit skewed to the low end, by which i mean bass is somewhat lacking and highs are a bit....high, but you can still accomodate for that once you get to know them just the same as the mids on the HD280s. This definitely makes them viable as a choice and could benefit you more on longer studio sesions.
Last but not least is an old time fav--Sony HDR7506s. Yes...its a long model name. If I didnt sell this stuff I wouldnt remember it either, but alas they are a studio classic. They are easy on the head with smaller cup size then the HD280s or SRH440s, and provide equal accuracy to the 440s. They are also a bit skewed to the lower end. They arent my go tos anymore but they are for a lot of people.
They are all popular headphones in the pro audio world so if you can find yourself a music store that lets you demo headphones and has them.....do so first. Then buy em from those guys too, cuz retail stores need the love. Plus most places will match internet pricing so you can bring in a sale price from another site....which is whats key to getting these under 100 bucks. They normally sell for right AT 100....meaning tax will get you over a bit likely. BUT....find a sale on any site for them for less, and wam bam...get the deal almost anywhere, stay under 100.