I applied to 5 schools, including 3 that used the OTCAS. The applications and the GRE together cost me about $400. Travelling to three different cities for interviews was more expensive--about $600, for a total of $1000.
The number of schools to which you apply would depend on the strength of your application and the number of programs that truly interest you. It would be beneficial to really investigate each program on many different levels (NBCOT pass rate, accreditation, teaching philosophy, curriculum, class-size, faculty, location, price), so that you don't end up wasting your money on an application for a program that you wouldn't realistically attend.