If i'm not wrong, semi-acoustics are simply acoustic guitars with pickups. Why they're called that is a total mystery to me. Some considerations:
Wood
The wood sets the tone of your guitar. Note that this is in general and not always the case. There are 5 parts: top, back, sides, neck and fretboard. The top affects your tone most, followed by the fretboard, the back, the neck and then the sides. This may not be accurate, its only a general guideline. Most acoustics have either spruce or cedar tops, spruce giving a more bright, punchy sound while cedar gives a mellow, sweet voicing. Back and sides are most of the time made of the same material, and there are many many many combinations. Check out
www.taylorguitars.com and go to the Features tab. Its a pretty good resource, if you take it with a pinch of salt.
For the top, back and sides, they are either laminate or solid. Laminate means that the top isn't a single piece of wood, rather several layers glued together. The glue affects the tone and resonance of the wood. Solids have to glue in between, hence are generally more resonant than laminates. For your price range, expect all-laminate acoustics to be within your budget. If you're lucky you can get a solid top at the very most, but even then you have to try it first, to see if you like it. Rule of thumb when buying a guitar: if it sounds good to you, its good enough, regardless of the features.
Shape
There are several shapes, with the more popular ones being dreadnoughts, grand auditoriums and jumbos. Again, the link above explains it in pretty good detail.
I suggest you try Custom Acoustics from Citymusic, Takamine D series from Luthermusic or Fina from Davis.