I don't have any great words of wisdom on these, but I did look tonight and saw a pair of Penta 3's on eBay for $1600, and another pair on eBay for $2000. That might give an upper bound for you on the Penta 1's.
I had the pleasure of listening to a pair of those back in the early 90's. I don't know which version they were, but I thought that they were excellent. The owner had them cranked, and I could put my ear right up to them without any discomfort at all - the distortion was very low.