I don't think you're going to have a ton of luck finding the combination of great view / romantic / moderately priced. You may have to shoot for 2 out of three.
Romantic / Moderately Priced / Good Food: I found Le Club Fez on James Island to have a great date night vibe. It's Moroccan / French food so it'll be unique in that regard, and entrees will run you around $20. That being said, it doesn't have any sort of a view, but it's also located next to Charleston's art house movie theater, so you have the 2nd part of a great date right next door. I wrote a full review of it when I took a friend there for her birthday a couple months ago: http://dhisgood.blogspot.com/2008/04/le-club-fez-restaurant-review.html
Great View / Affordable / Could be romantic: Bowens Island Restaurant in between James Island and Folly Beach has one of the best views in the area, looking right out over the Sol Legare Creek. You sit on plastic furniture and eat off of paper plates, but if you went a little later in the evening there would likely be pretty few people there and you'd be able to catch an awesome sunset right on the water. The food is local/casual, but it's a very Charleston experience and could fit your needs: http://dhisgood.blogspot.com/2008/06/bowens-island-restaurant-restaurant.html
Romantic / Moderately Priced: My last suggestion would be Pane e Vino on Warren St. downtown. It's a wine bar / restaurant with reasonable prices hidden away from all the touristy spots on the Market. I haven't written a review of it, but I've heard nothing but great things about it. You can read the City Paper's review here: http://www.charlestoncitypaper.com/gy...
Hope this helps!
If you're looking for a place on the water, I'd try the Boathouse on Sullivans (there's one downtown, too, so pay attention if you're making reservations). If you want downtown, McCrady's, Cypress, and Oak are all beautiful restaurants with great food. All of these options can be pricey, and are probably out of the $20-$40 price range if you include drinks. I'd suggested looking at the menu online before making a res. Meritage is a tapas restaurant with plenty of wine choices. It can get a bit loud and crowded, but it's a better price range and I've enjoyed dates there. If you go to a downtown restaurant, you should try one of the rooftop bars (Vendue Inn or Market Pavillion), or a walk around the battery to have a good view of water and some of the great houses in the area.