Each route has a big range of fares, with both advance-purchase discounts and roundtrip discounts, and can be more expensive if purchased on short notice. I don't find the prices that you quoted on Rail Europe, except for the Eurostar rate, although there are options in that price neighborhood. Some of them look closer to first class with advance purchase discounts, while the comments above are assuming and recommending 2nd class (which we agree is fine).
BritRail passes don't cover the Eurostar, so it sounds like you'd only use 2 days of a 3-day pass.
FYI, I see the BritRail South West pass for 3 consecutive days at $424 for 4 adults or the BritRail England pass for 3 consecutive days at $500 for 4 adults. (Consecutive would mean traveling Saturday and the following Monday, not Tuesday.) With either coverage area, the version for 3 days spread over a longer period costs a little more, but not exactly $473. Your agent may have added in the costs of a shipping fee, optional "Protection Plan," or their own fee.