At $300 it’s going to be very challenging to get a solid industrial walking foot, but not impossible if you’re willing to put in time waiting for the right deal. I would get the craigslist app if you don’t have it already and set up as many alerts for all relevant keywords: Consew, Pfaff, Juki, Singer, Brother, Sailrite, Seiko, walking foot, Industrial Sewing etc.
At that budget range you might be waiting for a while and if you do find something be sure to bring lots of heavy duty test material and run the machine for significant time to make sure it’s not a lemon. Bring a bottle of high quality sewing machine oil since many industrial machines often sit idle for years and can seize up when not regularly used. I have a Consew 206RB-2 walking foot and it was in serious need of oil when I got it.
A lot of industrial machines are often sold as just the “head” unit, so it’s possible to buy the head, then get an industrial sewing table to mount it (maybe $50 if you get a really great deal on CL) then mount a 1/3 or 1/2 HP clutch motor (crappy no name ones can be found for $100 but don’t expect much and they are challenging to control depending on how you gear them). That would leave about ~$150 budget for the head unit which would be an amazing deal for walking foot.
Are you sure you need a walking foot? What’s the heaviest material package you’re looking to run through the machine? For $300 to $400 you could probably pick up a decent industrial (non walking foot machine).
One last thought, pretty much every major city has at least a couple industrial sewing machine service places/retailers. Go sniff around there, befriend the owner and let them know you’re interested in getting your hands dirty but are on a tight budget. I’ve found the 3-4 I’ve befriended were always pretty open to cutting me a deal once they knew I was genuinely interested in the machines and not just trying to be cheap.