It depends on how your contract is set up.

Is this a custom home, or did you pick a pre-set package from the builder? If the latter, he probably has a flooring source that he uses and trusts, who give him a builder's discount and a good guarantee. If you buy your maple elsewhere and he installs it, he's on the hook for any unforeseen problems, such as: the flooring isn't delivered on time; or the finish is no good; or you need extra because the quality is poor and a lot of boards are culled out. Some builders won't do changes like this, some may charge you for it, and some may tell you to handle it yourself.

I can't be sure but I think maple is a more expensive hardwood than oak. Talk to your builder again and ask how you can come to a more agreeable price or maybe he can show you a stain over the oak that you would like and not be able to tell it's oak? I have seen some beautiful oak floors stained with a darker finish that were so pretty!! I was shocked to find out that they were oak floors!