Assuming it has a title I would jump on a $350 TW in a heartbeat. Sounds like it is more or less complete just needs some work. Worst case would be that it is a parts bike and you would likely get at least $350 out of it for the cost of a little elbow grease and some time on E-bay/Craiglist and the Classifieds here.
If you wanted to get it up and running to full street legal and safe the cost would depend a ton on how handy and creative you are and what parts it would need. A few of our class A scavenger and tinkerers on the forum have put TWs into functional service for virtually nothing but time. These are old style engines and carbs on a simple motorcycle, easy to fix and very tolerant of the shade tree mechanic.