Paulo
I have looked at a number of Pitts S1's which people have bought to me in response to an advert some while ago. IMHO, virtually everyone with an immaculate Pitts is deeply in love with their aeroplane and would sooner sell the kids than the a/c.
Most of the ones I have seen are dogs. They have been ******ed about with by DIY how can I save a few quid type owners, or are very tiered. Either way they need taking apart and rebuilding. The joy of a Pitts is that you can do this and come out with an immaculate a/c, but it is not cheap.
If you buy a shabby S1S (there are only three and a half T's in the UK) with an engine that is spray painted a whole batch of different colours (always worth avoiding), expect to pay very ball park 15 to 20K. Most owners want more but they are walter mitty types. Having the thing taken apart and put back together as per factory original will cost between 10 and 20k depending on what nightmares are uncovered, like for example, wing spa damage, broken wing ribs, broken tubes in the frame etc etc.
Once you have a really immaculate S1S expect 25 to 30K tops.
Remember, tho, a Pitts is not just for christmas it is for life and it will last a lifetime and cost next to nothing to run on a PFA Permit. I am biased as I have 2 which is greedy but hell you're a long time dead.