The 375K car is a marketing try related to its period "authenticity". I would think someone with 375K to spend probably knows about SPF, but you never know, and it does cost to get the details right as here. Seems like this would be a good Barett Jackson car.
As to the investment value of a "regular" spf. The used cars depeding on spec and age seem to have deprciated 20-30% and then flattened at that level. That is paltry compared to the depreciation of say a ferrari or a porche.
A SPF is a bargain, look at what another "real" Gt40 costs from gelescoe of Holman. My guess is with time the price from Sa will continue to rise, and production in Sa will cease or move to a more expensive locale.
New rollers will continue to get more expensive and less will be sold, the cost of completing a car rises every year too. As a GT40 does not really change, its not like there is a new model comming out, lower production and higher roller and completion prices will have an effect on used prices, as these are just a comparison to new prices.
The day is apparoaching where a really Good SPf is going to cost 200k to complete. That means a used one is going to be 135-150K depending on spec, condition and to alesser extent age. What helps here as well is limited supply and a few dealers controlling inventory. Of course more cars built can mean saturation, but the numbers are still really small. There are far more Ford Gts about and their price rises.
Of course if SPF ceases production then you have an instant 300k car.
Even still in todays market right now buy a SPF use it for 10 years take a 25-30% hit, not a bad deal, and its not like they break the bank to keep running. A 10 year old ferrari is a 50% or more hit(in $ terms more than the cost of a spf), and the running costs are another 50k on top of that for 10 years, plus a 10 year old ferrari is obsolete, whereas a 10 year old SPF is still a classic and worth keeping, its appeal has lasted 50 years and grows..