The cheapest you're gonna find is to build your own with templates located on this website. Not sure what your skillset is but if its within your ability to do I highly recommend it,
That $300+ range seems to have been the going rate for gaunts for years. Just remember that you do get what you pay for when looking for bargains. Most sellers on this site are probably safe, but keep this in mind when searching eBay.
If you do build your own you can usually contact some of the regular gauntlet makers to purchase casts of some of the harder/"fiddlier" parts such as rocket, MQ keypad, dental expander etc.
Hope this helps! Good Luck!