In today's market a lot of developers are financing the construction of their projects using the deposits from their buyers. As a spec home is conditioned to your liking and the developer will need to do a lot of upfront payments for the permit fees, underground utilities, impact fees, etc., it is not unreasonable for them to ask for the 10%.
What I suggest is that you conduct a background on the developer and have an attorney review the contract.
Also, make sure that timelines are clear on the contract such as duration of construction, etc. so that the developer does not take 2-years to build a home that on average should take 4 - 6 months.