Is that the one that basically has a plenum spacer built onto the manifold to increase the height? If so I'd say at LEAST the cost of just a plenum spacer ($200 or so). Never seen dynos of the manifold so I don't know how effective it was which would affect the price. Polishing is highly subjective and depends entirely on the type of polishing that was done. A high end mirror polish job can add $300 or so to a premium piece of equipment, a quick silica blasting might not add any value or might actually detract if any raised lettering was rounded off and looked wierd. A DIY without going through the proper sanding papers to remove sanding lines will DEFINITELY detract from the price because it looks amateur.

If it looks good I'd say somewhere around $300-500 or so? Never seen one for sale so who knows, if there are substantial gains with the manifold which there PROBABLY are since it likely was part of that $10k TT kit they sell then it might sell for more than that.