Fares from London to Cambridge should be about £20/person (return), so slightly less (not half) for a single. (...) What were you quoted?

The quotes are in USD, and I'll give totals because this includes all three of us (me, Jenny, Rachel).

"Heathrow express"... $87.00. I'm not sure what this is. I assume this is getting from the airport to our hotel or something. I dunno, I have to ask the agent.

"Britrail Days Out Pass 2 days within 8 days"... $209.00. I believe what the agent did is quoted some kind of a bulk pass on the rail system that we could use any time, rather than just a one-way point-to-point ticket to Cambridge. I think it's because she wasn't certain when exactly we were going to Cambridge.

"Eurail Selectpass 5 days within 2 months"... $760.00. I think this might be a similar thing to get us between Amersfoort and Germany, and within Germany for the week we'll be there.

I think most of the above is probably not needed, and there's easier and cheaper ways to get between our destinations...