The plan is if you get on at SR 13 and off for US 17 then there is no toll. The bridge span length determine the cost, with the exception of the main canal. A "signature bridge" cost would be out of site. As it is this crossing is around $300 million.
The new bridge over the Ohio from Kentucky to Indiana in Louisville is forecasted
at $1bn. That was for either a cable stayed tower approach or arched.
A similar project for the new I-69 bridge over the Ohio at Evansville came in at $1.4bn.
But these are significant spans traversing larger channels than the St Johns and include the cost for approaches due to such a large flood plain.
But the new Stan Musial Veterans Bridge in St Louis (the "Stan Span") over the Mississippi got built for $346 Million after going through some cost reviews. That excludes the approach costs which were costed seperately.
The new Louisville span was priced at $238 Million excluding approach costs.
So I think it is possible to be iconic based on the geography and the budget.