If I had enough money to cover the costs of the frame & body along with a Covette C5 for running components, I'd love to take a shot at building a Factory Five GTM (https://www.factoryfive.com/gtm-supercar/) -
The GTM Body & frame starts at $25K + options plus the cost of a C5 Vette in whatever running condition is acceptable to the builder. A neighbor in my community built his own Cobra replica over a two year period.
Before I purchased my Slingshot, I considered having my Daughter help me build a different Factory Five car using the running gear from a Subuaru WRX, the 818 - https://www.factoryfive.com/818/. The 818 body/frame starts at $9900 for the open top model or $13K for the coupe + cost for donor WRX. A frugal builder should be able to build the open top version for around $20K or so, less if he/she already has a running WRX.