Fair enough on the onsen.
And the tickets depend, really. That site Hyperdia does tell you how much if you type it in, and the prices are prettty accurate. Give it a dummy run.
The question you should be asking more is, 'do I need a JR Pass'?
Long story short, if you are going to be travelling between towns a lot, then probably. It's a big outlay of money, but repeated shinkansen trips can easily start to stack up. However, be aware that not every train or shinkansen is covered by the JR pass, and it is routine to pay for a few trains on top of that. But it is definitely money-saving if you are travelling quite a lot of distance.