You cannot use a GoPro for shooting anything that will look remotely professional except what it is intended for, i.e. unattended video in physically challenging locations. It has a fixed wide-angle lens and precludes getting any depth-of-field effects. Shooting with a cellphone camera is a joke -- you have no real control over the video, which is going to be of relatively poor quality. "HD" refers to resolution as measured by pixels, but says nothing about compression. Cellphones shoot highly compressed video, which results in poor detail, pixelation and digital artifacts.
You can get camcorders in your price range, but these are not designed for artistic shooting, but for recording family events, chasing your kid around, etc. You can get DOF adapters, focus followers and other add-on kludges that will allow for cinema-like shooting, but you'll wind up spending twice what you spent on the camcorder.
Someone mentioned DSLRs that shoot video, which are good options. They provide for DOF effects and there are free firmware add-ons available, e.g. Magic Lantern (for Canon) and CHDK (for Nikon), that let you shoot RAW video, which is necessary for proper grading (matching color and gamma from shot to shot). To use them properly, you have to shoot full manual (no autofocus or autoexposure) and a good tripod with a fluid head is a must. A good tripod/head combo will cost your entire budget.
Finally, though, consider this: Saying, "I want to shoot music videos and have a budget of $400," is like saying, "I want to record albums and have a budget of $400 . . . and know absolutely nothing about music or recording, mixing and mastering audio." Shooting pro-quality video requires a thorough understanding of the hardware and software, lighting and composition, just as producing good home recordings requires a thorough understanding the hardware and software, and such esoteric subjects as mike placement, room treatments, and arcane technical topics like phasing, filters, etc. I've been shooting video as an amateur for around 20 years. I've spent thousands on gear and thousands more on software for editing and post-production. I use mostly semi-pro cameras and spend weeks (and sometimes months) to produce a 3-minute sequence that I think is good enough to put on YouTube. And, I guarantee, though I think my videos are pretty good, no one is going to mistake them for professional production. I've been into home recording even longer, and spent comparable amounts on recording gear, hardware and software. I use home recording to produce "internal demos," i.e. demos to give to friends for feedback but NOT for public consumption. Again, though, for the purpose I use them I think they are very good, no one is going to mistake them for professional production.
It takes a considerable amount of skill and education (not to mention musicianship and artistic talent) to produce pro-quality audio and video. A pro can turn out good work on even the most mediocre gear. However, someone who lacks a pro's skill-set will turn out junk with even the most expensive and elaborate production equipment at his disposal.
I'm not saying this to discourage you from trying to make your own music videos but, rather, to get you to set your expectations appropriately. Your question, alone, is evidence that you know next to nothing about video. That's fine, and there's nothing wrong with asking questions (and everything right). Just don't expect to buy something, spend a week or two fooling around with it and come up with something that compares to what you see on MTV.
You're in the right place to learn about home recording -- despite having had a home studio for more than 20 years, I am constantly picking up new and valuable techniques here. If you listen to the work of some of the more highly regarded artists on this site, you'll be amazed (in a good way) by the quality of the work, both as technical recordings and artistic expressions of musical ability.
I'd recommend you visit dvinfo.net, which is the video equivalent of homerecording.com, and learn a bit about video before laying out hard-earned money for gear.