Good audio isn't cheap, but there are plenty of inexpensive options to get you much of the way there without breaking the bank. Here's Caleb Pike from DSLR Video Shooter with some great budget audio options, all for under $250 each:
These are the three setups, with a full parts list over on Caleb's site (it looks like the mic stand he recommends may be out of stock, so there are some other options available that should do the job):
- $129.05: RØDE VideoMic GO + Extras
- $229.04: RØDE VideoMic GO + Zoom H1 or Tascam DR-05 + Extras
- $131.26: Zoom H1 or Tascam DR-05 + Extras
Obviously some setups will be better than others in terms of what you're trying to record. The Zoom H1's internal mic won't do very well if you've got to record from a distance — that's where you'll need the VideoMic GO. If you're recording interviews or the subject is relatively close to the camera, these setups are all going to be good if you don't have a lot of money to spend.
If you do have a little more money to spend, moving up to a true shotgun mic like something from the RØDE NTG series and possibly an audio recorder like the H4n or H6 will give you more options and better quality. Tascam also makes some great affordable handheld recorder/mics. If you need something with a wider pickup pattern that's more configurable, RØDE also makes the Stereo VideoMic X, which will mount on your camera (though it's a pit on the pricier side compared to these other options at $800).
These more expensive options will take you into the $400-$600 range depending on what you're looking at, but if you do have a higher budget, there are some terrific recorders that will give exceptional audio from both of these companies. Unfortunately, once you move into a certain style of recorder, that will also make it more difficult to operate both camera and audio, so if you're by yourself, try one of the smaller recorders from Zoom or Tascam first, and if you have a little money to spend, springing for a better microphone will help you get better sound from a distance.