I don't think there will be any particular value derived from the kit manufacture, in these cases, but rather, you'll want more to set the price according to the quality of the finish. The price of Airfix and Historex kits is highly speculative, but I'd venture a range for an unfinished kit from $5 to $15. But for a painted figure, it's going to depend on the quality of the finish. Also, connoisseur collectors these days are more likely to look down their noses at these old kit figures, unless it's a really outstanding job of finish.
Prost!
Brad