Deciding how to stretch your budget depends on your personal specialty preferences and priorities. If finances are a factor in your application process, it's always good to pick a "primary" specialty and a "secondary" specialty. You can focus on the primary and use the rest of your resources on the other specialty. A general guideline for applying to programs is you should aim for a minimum of 100 programs per specialty (upon availability of compatible programs). In your case, Neurology is a smaller specialty, so you may naturally end up applying to fewer programs, and using the rest of your budget on Internal Medicine.
Budgeting your application also means being aware of all of the fees involved in residency, to learn more, you can view all of the fees at: http://blog.matcharesident.com/fees-for-the-2016-17-us-medical-residency-application-season/