I agree with the last post. I travel frequently on Eurostar between Paris and London. Typically, the fares are much like airline tickets. Only a certain number of seats are allocated at lower rates. How to get these rates would include the following - do not travel during typical business travel days and times - Eurostar train is a main means of transport for the business traveller. So forget Monday morning and Friday evening for example. If you go to www.eurostar.com - you can play with dates and times on the schedule and begin to see the trends. You should purchase no later than 3 weeks out - this is when the tickets tend to jump.
Also - just a note about time zones - there is a one hour time difference - it is best to keep that in mind when you are debating your trip.
Lastly - I would not waste the extra money for First or Business Premier - the food is not worth the additional fee. Second class is roomy and comfortable - buy a picnic for yourselves and enjoy.