As much as a collector is willing to pay, with due regard to rarity and condition. Try a web auction site and set a reserve price below which you are not willing to sell.
The answer depends on your own country's economic circumstances, but here in Australia the "cost of living" has increased by about 520% between 1974 (when the TV series began) and 2006. This is based...
The TV version had telescopic and infra-red (night) vision and super-human strength in his arms and legs and the ability to run upwards of 60 mph. The version of the character in the original novels...