When considering these XP runs of ???-CDG-AMS-??? rtn or ???-AMS-CDG-??? rtn over a weekend for about £500 that gives about £5.5 per XP. But these weekend XP runs then have to include a hotel stay and base airport (LHR) weekend car parking. Which increases the £ per XP for a weekend.
What are the best (if any) XP runs that can be done in one day to avoid the hotel cost and lower car park fee? For me I guess it can only be LHR-AMS-CDG-AMS-LHR for 60 XP but not finding them.
That’s true, however most airports in Europe will offer “cheap” (relatively speaking) accommodation (usually at or around EUR 50). Once factored in, you can still get good deals even with an overnight stay.
You can also get lower than GBP 500 mileage run deals in J yielding 60 XP. Try to book > 30 days in advance, and you can get deals as low as EUR 380 (I’m doing for this price FRA-CDG-ZRH-CDG-FRA). Ex-LHR is notoriously expensive for Intra-Europe flight on Skyteam (I guess Eurostar may have something to do with it), but you can still get good deals at around GBP 380 to GVA or FRA through CDG. But you need to play around with dates a bit. If you want your itinerary, try perhaps to “nest” it onto 2 itineraries, but unlikely to find such prices.