I made the same mistake when I calculated a short stay rental in February (the weekly rate was reduced for last minute bookings). I forgot ( ) that any special availability discounts we have does not apply when calculating short stay breaks.
I did however let the guests know my mistake and as it was my oversight I didn't ask them to pay the difference. The amount I overlooked was £75..small enough for me to shrug it off and regard it as a PR exercise!
Took a 2 week booking for September quoting them £600 a week so they paid the deposit of £300, only then did I realise that it should have been £800 a week!! It was months ago now and I've since put those weeks up to £1000 so all in all I'm £800 down - that might make you feel a bit better Elaine!
Had our property at a discounted rate on one of the sites, very nice man phoned up to ask if it was free 11-19th April, I said yes, was that for a week and he said yes so I duly sent him a contract for 11-19th April for quoted price per week. After I sent it I realised actually it wasn't a week, it was 8 nights (though he had said it was a week) so I rang him to just check the dates and when he confirmed it was 8 days, not a week , I said i would pro rata it so it was an extra £20 for the night (yes we are cheap in this area!!)
He cancelled the booking!!! But I am getting very hardened, he was having it cheap anyway and if he can't pay the £20 extra I would rather it was empty!!
I think I was right? (though I should have double checked before I sent the contract)
After I sent an open-ended e-mail explaining the confusion and apologising, but not specifically asking for the full original amount, they paid up the full balance without a whimper! Checked them in and they are adorable people! There are still honest people in the world. (and they're now enjoying some excellent wine on me as a peace offering!)
Thanks for all the advice and the differing views - they really helped me to work out what I should do.
Elaine
) that any special availability discounts we have does not apply when calculating short stay breaks. 
It was months ago now and I've since put those weeks up to £1000 so all in all I'm £800 down - that might make you feel a bit better Elaine! 
..but I would have taken the booking and overlooked the £20 rather than have the property empty....(just my humble opinion). 
(and they're now enjoying some excellent wine on me as a peace offering!)