I think starting at £3.40 for pots seems about right. That would put them within the price range of most people, but still allow a bit for profit. The gold fish bowl in Oxford used to do tropica plants buy they started at 7 or 8quid, aint no way Im paying that! I dont care who grew them!
Personally for things like HC and more specialist plants I'd be happy to pay 4 to 4.50 per pot, mostly as I knew they were tropica and therefore the quality would be excellent. Greenline do HC pots for £3.75 and the quality was good (although just short of tropica but still good) but the pot size was much smaller, say around a half to three quarters of the tropica pots.
Sam