How much? Give me an exact guessed percentage. Say a site was appraised at domaintops.com for 7000 USD and later someone added 240 backlinks. How much would the value increase? Of course, you have to factor wether the backlinks are valuable.
Very little. It's a simple enough task to add backlinks. Go check out the services section of this very board and there's people there who will add thousands of backlinks for a few bucks.
However, if backlinks are added and the SEO is done right then the site goes to the top. If it goes to the top then big money.