How to value an I phone game?

Does anyone know of methods in establishing the value of an i phone game?

I have a client who has developed an i phone game and a publisher has conservatively estimated that it will be able to generate income in excess of £250,000. It would therefore obviously advantages to incorporate and sell the game to the limited company. But at what value. The estimated income may be in some extent trying to secure the publishing but it has some credibility in that they are prepared to pay an advance of £25,000.