As much as I like to save money, comparing lake magic to the Fort is apples and oranges. On some level you have to decide why youre even going to Disney ? I could just go to a local amusement park, its not the same as Disney but its cheaper.
I don't think anyone will disagree that there's a significant difference between Lake Magic and Fort Wilderness but there's also a huge difference in price. I think you can usually get a site for $40-$60 (or less) per night at Lake Magic (I believe they accept Passport America, Good Sams and
AAA). If you are going for several days or make several trips per year, it might be worth looking into Thousand Trails membership (they have many different ones so do some research if you are contemplating this). For under $600, you can get a SouthEast Zone membership pass which will give you unlimited up to 14 days stay (followed by 7 days not at the resort) at Thousand Trails (e.g. we typically come to the area 3 times per year for 1 week so for us, it works out well and from my experience, it's easier to get a site if you are a member (e.g. if you call for "regular" availability, there might not be any but they have sites reserved for members available). (e.g. when we got our membership last spring, there was no availability for a "regular client" but by joining for around $560 (I think), we got a site for 13 nights and then we used the membership again this past XMas for 9 nights so so far, for $560, we've stayed 22 nights (equals roughly $25 per night).
Back to the original question, some have already been mentioned but the following campgrounds are nearby and are modestly priced:
Tropical Palms
Sherwood Forest
Outdoor Resorts Orlando
Lake Magic
Thousand Trails Orlando
Lake Louisa State Park
and others if you care to drive a bit more
If you search the forum, this question often comes up and you can find more related discussions.
Hope this helps