No, this isn't at all too much to ask for. Many such deals are done here in California using 1% to 5% of the agreed upon purchase price as a non-refundable "option fee." You'll also need to define whether or not the option fee, and any lease payments, will apply in whole, or in part, to the ultimate purchase price.
Just make sure you have all the conditions spelled out in the Lease-Option Agreement AND the Purchase Agreement.
May 27, 2009