Thanks! I think I've mostly watched resin casting demonstrations by very experienced people, so I may have unreasonably positive expectations of what is possible, but I think it's fairly common for art students to get some successful resin casts in a single-semester sculpture course? Which I think implies a cost of first success well under US$10000? As I said, I haven't tried it myself. Michal Zalewski makes it sound pretty achievable.

As far as thermoplastic molding goes, aside from the vacuum forming that someone else suggested, there seem to be a lot of experimental options. There is a YouTuber who has built a couple of DIY injection molding machines (http://www.youtube.com/user/AndysMachines), and Star Simpson did successfully mold recycled HDPE plasticized with vegetable oil in an open-top mold (https://www.instructables.com/HomemadePlastic/), but the results are not super smooth. My recollection from melting plastic cups in the oven as a kid (probably polypropylene) is that you end up with a puddle with a pretty flat and smooth surface except for whatever lettering or imagery was on the outside of the cups, so I feel like you might be able to get a smooth top out of an open-top mold, but I don't know which plastic that was, and melt degradation might be an issue. https://youtu.be/KA1UELu33FI shows some apparently successful open-top molding with powder made from recycled polypropylene (skip to the last minute of the video) but doesn't test it for strength. https://www.youtube.com/watch?v=2OYbyzgErvo recycled polypropylene into a two-piece water-cooled mold improvised out of aluminum baking tins, but seems to have suffered major melt degradation possibly due to overheating it in a can on an open fire. https://youtu.be/Ae8_OWju8RM recycled (probably heavily filled) HDPE into a billet in an aluminum can in a thermostat-controlled electric oven at 180 degrees and had a lot of porosity due to high melt viscosity, then turned the billet down on a lathe; the outside of the billet seems to have suffered enough melt degradation to make it machine much more cleanly than the center, which produces the kind of long stringy chips you'd expect from HDPE. https://youtu.be/blSvUqfuiuo makes an HDPE sort of gingerbread man in an open-top silicone oven mold, the kind you'd use to make gingerbread-man-shaped muffins, suffering melt degradation severe enough in parts to turn the HDPE brown, warping, and significant porosity due to high melt viscosity. https://youtu.be/-igxhoGEQFU shows really nice marbled results melting HDPE in a panini press with a silicone mold release, then pressing it into a billet in a particleboard mold with a bench vise, but still suffering some problems with porosity. By turning it on a wood lathe they are immune to problems of warping etc.

Polycaprolactone deserves a mention because above its glass transition you can mold the thermoplastic with your bare hands, but pouring it into a mold is out of the question because of its high viscosity, and as a result of that low Tg you tend to get a lot of creep, and parts become unusably weak in a warm room.

So I think you've convinced me that thermoplastic casting without injection molding equipment is not a practical option for making low-volume parts. Thank you for sharing your experience!