“Mother's Love" is built mainly of pressure treated wood, with coated steel fasteners. Since the wood is isolated from the elements, it will last practically indefinitely. On top of the wooden structure are layers of steel mesh and concrete. In concept this is similar to the methods used to make concrete boats (ferro-cement). The thin shell concrete is not only there to help support the structure, it also makes a permanent, weather and vandal proof structure, all by itself. Even if it doesn’t get re-painted in forty years, the gray concrete sculpture will still be there, indefinitely.
The fabric-covered models will only last as long as the paint. Then the fabric will deteriorate, leaving the wire mesh and wooden structure, abstract maybe, but not too attractive to most people. These are mainly just for short-term installations, where the sculpture will have to get moved again. Still, the strength of the wood itself should be sufficient to resist a 90 mph wind, and the fabric is coated in fireproofing paint to resist vandalism.
For anchoring it to the ground there are a variety of recourses, including buried piers, and ground anchors, which are essentially giant screws driven into the soil. If you think of the structure as a kind of sail, or chimney, it will give a better idea of the wind stresses that will be encountered.
The hands and face are made of cast resins, which have a good service life when exposed to sunlight. If they are kept painted (or preferably gold leafed), they will also last forever.
Sculptures are like any other structures built of similar materials. They suffer from sunlight, wind, rain, vandalism, and bird poo. The sad facts of life follow you around no matter what you do in the construction industry. The cost for the 30-foot high statues will vary between $60,000 and $120,000 depending on the details. Pretty obviously, gold leaf on a full set of hair will be more than gold paint, without the hair. There are a lot of other details as well. The distance the proposed site is from my home base here in Virginia, for example, is bound to make a difference, as will each state's labor laws, etc.
This price is extremely cheap considering the competition, and the quality. A limited edition four-foot high bronze statue on the art market might sell for $50,000. I don’t even know what a 30-foot metal statue would run. However, copper cladding could be added to the outside of the "Mother's Love" statue, and then be given a patina for color, if desired.
Most public art statues cost a lot more than this. Washington, D.C. recently put out requests for $400,000 in public art statues, on three different sites.