Easy;
Tell her to pack an overnight bag but dont tell her where you're taking her to. Tell her parents and a couple of her friends who you know will tell her anything you say, even if you ask them not to (you know the kind "hey dont tell him i told you so but he told me where's hes taking you!"), that you're suprising her with a nice hotel room and dinner just dont mention in what state the hotel is in , book flights and accomodation at Daydream Island (or similar). Try and get a room like this if you can;
Have a nice bottle of wine waiting on arrival and if you feel like it, 12 roses. Although the last part may be a bit over the top that would depend on you and her. If she likes material things etc on the last night give her a necklace or something meaningful.
She wont forget it.
(Btw, not exactly an "affordable/cheap" option, but who cares, at least you get a short holiday and some time in the sun)