Okay, first let's clarify terminology; there is no such thing as a boom microphone, although there are mics that are more commonly used on the end of a boom-pole, such as a shotgun mic.
As always I recommend hiring a pro. Next would be finding a knowledgeable, ambitious up-and-comer. If you are only shooting two or three times a year you really should rent. However, if you are dead set on purchasing your own audio gear...
The audio implementation of DSLRs is seriously substandard so you should strongly consider using an external audio recorder. You will have better control over the audio signal and be able to monitor the audio signal. A current budget favorite is the Tascam DR-100mkII, around $370. The ultra-low budget alternative that still has XLR inputs is the Tascam DR-40, around $180.
As far as shotgun mics are concerned the Audio Technica AT875 ($175) and Rode NTG-1 ($250) are good choices. However, since the audio recorder will be supplying the phantom power you will use up batteries at a terrific rate, or you will need to purchase battery packs and a charger. The alternative is to get the AT897 ($230) or NTG-2 ($270); the issue here, however, is that mics that supply their own phantom power via battery have significantly lower volume output levels, so when you increase the gain on the audio recorder you also add noise from the pre-amps.
If you will be shooting predominantly indoors you may want to consider a hypercardioid mic instead of a shotgun. The Avantone Pro CK-1 ($150) is a passable ultra-low budget choice. The Audix SCX1/HC ($500), AKG Blue Line SE-300B/CK93 ($480) and Audio Technica AT4053b ($600) are very good low/mid-budget choices.
You will still need headphones, a boom-pole, a shock-mount, wind protection and cables; these will cost as much or more than the mic and audio recorder.
But just having the gear will not give you good sound; you, or rather, the person running the audio, still need to know gain-staging and proper boom technique amongst other audio skills.
