I did a quick search on raileurope.com and it is 7 hours each way. You will be spending 14 hours in 3 days; My guess is that bus service is even longer.
The cost is around 170 US$ each way
I also checked on scnf.com which is the french website I use to buy tickets and the cost is 230 euros round trip which is similar to the 340 round trip US$ on raileurope
I would buy the tickets when you get there.
I think you will not find any cheap fares at this time.

Why not fly? If you leave saturday and return tuesday the round trip flight is 300 euros with lufthansa; there may be cheaper airlines

Could not figure out the OBB travel portal

I have purchased etickets from scnf.com and printed them at home; did not validate and just showed them to the conductor.