Audio, like the rest of filmmaking, is a unified whole from beginning to end. Despite the fact that hours are spent on the cinematography unless you are making a silent film the bulk of the story telling process takes place with the dialog. But even Oscar winning rerecording mixers using the best noise reduction plug-ins cannot save truly bad production sound, and you will not have near the tools or experience to fix even moderately bad production sound. If the production sound is really bad you have to resort to ADR, but most directors and rerecording mixers prefer to avoid ADR if at all possible; only the most extraordinary actors can match the performance/passion of the on-set performance with perfect lip sync. The rest of the time you settle for what you can get which detracts from the final product.
So think about sound during preproduction, take great care to capture solid production sound, then expend the time and effort during audio post.
Now, if you just want to learn and are not concerned about getting into festivals or obtaining distribution you can get a "cheap" set-up like the Rode VideoMic Pro and the Zoom H2, and make a DIY boom-pole. That would be around $400. The VMP will be relatively forgiving compared to "pro" mics and will sound much better indoors.
BTW, you may want to check out local music recording engineers; someone may want to venture into the world of production sound and/or audio post - and they should have a few decent mics already! That's how I got my start in film work; I migrated from music engineering to audio post and did some production sound work during the transition.