Site Development

Folks
A quick question which is not strictly architecture related.

A business acquaintance has been approached by a developer seeking to purchase his .5acre site in a large provincial town.

The site is zoned town centre, adjoining buildings are a mix of retail and apartments 5-6 storey’s high, and purchasing the site will greatly enhance the development potential of the adjoining land which is also owned by the developer.

The developer has offered a substantial cash amount but the land owner wants part of the retail/apartment space which will be developed on his site instead and has asked me what is the % norm of space for the site owner to demand in this type of transaction?

Any ideas?
Thanks
J