Covenant - Building on Land

I have a client who is in the process of selling green belt land (probably post 5/4/08) and they want to include within the agreement a 25 year covenent saying if the land is built on and sold within that 25 years, then they are to receive a share of those profits. They have asked me what percentage of profits they should go for - ie what would be normal in such circumstances.

Any guidance would be much appreciated.

Thank you.
Paul Booth