Great that you have a project to get stuck into, If you consider the cost of Ipe and an approxinate cost of £12k-£14K plus vat for the whole project; to then reduce the sq m cost of Hardwood deck by even £10 per sq m would only save you £600.
First Grade S4S/E4E KD Random Length Ipe currently (Jan 2012) is between £60 and £80 plus vat per sq m
To answer your question you could consider Massaranduba at Approx £52 per sq m or even Kurupay at approx £55 (harder than Ipe) or even Merbau (approx £72)
Do let you client know that they will all go silver eventually… or ask Thomas Rathbone to provide a quality Maintenance service to keep the colour.
I did write a blog about what decks cost here https://deckingnetwork.com/decking-advice/how-much-should-i-be-paying
Hope this helps