I seriously doubt you'll find an RC-model sized turbine for anywhere near $300, kit or complete. Perhaps some old JPX's or GW turbines out there for $800 used or so; I did see one JPX260 sell for $600 on Ebay a while back. Everything associated with turbines is going to be fairly expensive - airframes, support equipment, etc. remeber that if you plan to fly under AMA rules and insurance coverage that you'll need to fly a certified motor and get signed off by a manufacturer's rep.

There are a LOT of websites out there that deal with making homemade turbines from old car and truck turbochargers - if you just want to experiment around. But you'll still end up spending more than $300. Do a Google search on "home built turbine" or " home built jet engine".

Good luck (and don't hurt yourself!).