Just been searching some dates to either do a day trip to CDG or AMS within the first 8 months of 2010 w/AF or KLM.. and breaking up the journey to fly back into another airport. Nearly every single ticket I can see is over £300 one way.
I guess they're notoriously this expensive?
KL/AF generally have fare rules which mean the cheapest fares are only available for Saturday night stays or stays of 2 nights or more. Otherwise you're on "standard" economy fares, and of course their 1-way fares are quite ridiculous.
You might find that booking two returns works out cheaper than one return, and then just discard (or rebook) the other legs.
LH usually don't penalise you so heavily for doing short trips, and their E fares (the £49 o/w ones) are very useful for that kind of thing. If only they flew from Bristol.... (Not sure if SN have falled into parent company line on that one, but the SN fares I keep seeing just seem to have adopted LH's high YQs, rather than cheaper base fares).