What is the cheapest I could find a Testarossa for?
That is an extremely vague question. What year are you wanting? Do you want a flood vehicle? Do you want one needing the 30K service? Do you want one that needs body work? Color? If you can only afford the cheapest example, you won't be able to afford the maintenance, believe me.
I think I found one of the cheapest http://cgi.ebay.com/ebaymotors/Replica-Kit-Makes-512TR-Ferrari-512TR-Pontiac-Fiero-Replica_W0QQcmdZViewItemQQcategoryZ7251QQitemZ4634488069QQrdZ1
Not a TR guy (I have a 328), but I think what everyone is trying to tell you is that a cheap Ferrari is almost always a money pit. You might look at FerrariAds here, also DuPont Registry, and see what some of the higher mileage ones are listed at. The early ones (Testarossa) are going be le$$ than the 512TR that came later, and the F512M is a six-figure, rare machine. Also take a look at the maintenance costs. You can burn through $8K pretty quickly if it's due for a major service and has any "as long as we have the engine out..." incidentals. I like the 512TR but decided the V8's were a better way to start in Ferraris. Now that I have a 328, I'm in love with the car and may defer a TR till who knows when...
Within the last few months I notice 4 TRs sold in this price range. All were 90-91, all were from list members with exceptional maintenance records, recent major service, some including clutch. These were the cream of the crop. 2 91 white, 1 91 red, 1 90 yellow: $56.6K - $62.2K - Nguyen
Just sold my '91 TR with 33k miles and full maintenance records with no major service due for a while for $56103