The running cost of Hiller 12C are hard to say. Because it's old technology it's not predictable. For continuity a Schweizer 300 is a better way to go. However flying on the Hiller is more fun and it's a 'real' helicopter. If you buy a very nice refurbished Hiller and maintain it very well you get a lot of fun out of it.
I fly both by the way, and have a quite a lot of experience on both.